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The shortest-period binary star system known to date, RX J0806.3+1527 (HM Cancri), has now been observed in the optical for more than two decades. Although it is thought to be a double degenerate binary undergoing mass transfer, an early surprise was that its orbital frequency, $f_0$, is currently increasing as the result of gravitational wave radiation. This is unusual since it was expected that the mass donor was degenerate and would expand on mass loss, leading to a decreasing $f_0$. We exploit two decades of high-speed photometry to precisely quantify the trajectory of HM Cancri, allowing us to find that $\ddot f_0$ is negative, where $\ddot f_0~=~(-5.38\pm2.10)\times10^{-27}$ Hz s$^{-2}$. Coupled with our positive frequency derivative, we show that mass transfer is counteracting gravitational-wave dominated orbital decay and that HM Cancri will turn around within $2100\pm800\,$yrs from now. We present Hubble Space Telescope ultra-violet spectra which display Lyman-$\alpha$ absorption, indicative of the presence of hydrogen accreted from the donor star. We use these pieces of information to explore a grid of permitted donor and accretor masses with the Modules for Experiments in Stellar Astrophysics suite, finding models in good accordance with many of the observed properties for a cool and initially hydrogen-rich extremely-low-mass white dwarf ($\approx0.17\,$M$_\odot$) coupled with a high accretor mass white dwarf ($\approx 1.0\,$M$_\odot$). Our measurements and models affirm that HM~Cancri is still one of the brightest verification binaries for the Laser Interferometer Space Antenna spacecraft., Comment: 12 pages (+5 pages appendix), 9 figures, 6 tables. Accepted for publication in MNRAS

We present fast (~200s sampling) ugriz photometry of the low mass AGN NGC 4395 with the Liverpool Telescope, followed by very fast (3s sampling) us, gs, rs, is and zs simultaneous monitoring with HiPERCAM on the 10.4m GTC. These observations provide the fastest ever AGN multiband photometry and very precise lag measurements. Unlike in all other AGN, gs lags us by a large amount, consistent with disc reprocessing but not with reprocessing in the Broad Line Region (BLR). There is very little increase in lag with wavelength at long wavelengths, indicating an outer edge (Rout) to the reprocessor. We have compared truncated disc reprocessing models to the combined HiPERCAM and previous X-ray/UV lags. For the normally accepted mass of 3.6E5 solar, we obtain reasonable agreement with zero spin, Rout ~1700 Rg, and the DONE physically-motivated temperature-dependent disc colour correction factor (fcol). A smaller mass of 4E4 solar can only be accomodated if fcol=2.4, which is probably unrealistically high. Disc self gravity is probably unimportant in this low mass AGN but an obscuring wind may provide an edge. For the small mass the dust sublimation radius is similar to Rout, so the wind could be dusty. However for the more likely large mass the sublimation radius is further out so the optically-thick base of a line-driven gaseous wind is more likely. The inner edge of the BLR is close to Rout in both cases. These observations provide the first good evidence for a truncated AGN disc and caution that truncation should be included in reverberation lag modelling., 19 pages, 27 figures

Accreting magnetic white dwarfs offer an opportunity to understand the interplay between spin-up and spin-down torques in binary systems. Monitoring of the white dwarf spin may reveal whether the white dwarf spin is currently in a state of near-equilibrium, or of uni-directional evolution towards longer or shorter periods, reflecting the recent history of the system and providing constraints for evolutionary models. This makes the monitoring of the spin history of magnetic white dwarfs of high interest. In this paper we report the results of a campaign of follow-up optical photometry to detect and track the 39 sec white dwarf spin pulses recently discovered in Hubble Space Telescope data of the cataclysmic variable V1460 Her. We find the spin pulsations to be present in g-band photometry at a typical amplitude of 0.4%. Under favourable observing conditions, the spin signal is detectable using 2-meter class telescopes. We measured pulse-arrival times for all our observations, which allowed us to derive a precise ephemeris for the white dwarf spin. We have also derived an orbital modulation correction that can be applied to the measurements. With our limited baseline of just over four years, we detect no evidence yet for spin-up or spin-down of the white dwarf, obtaining a lower limit of |P/Pdot|> 4e7 years, which is already 4 to 8 times longer than the timescales measured in two other cataclysmic variable systems containing rapidly rotating white dwarfs, AE Aqr and AR Sco., 8 pages, 6 figures. Accepted for publication in MNRAS

AM CVn systems are ultra-compact, helium-rich, accreting binaries with degenerate or semi-degenerate donors. We report the discovery of five new eclipsing AM CVn systems with orbital periods of 61.5, 55.5, 53.3, 37.4, and 35.4 minutes. These systems were discovered by searching for deep eclipses in the Zwicky Transient Facility (ZTF) lightcurves of white dwarfs selected using Gaia parallaxes. We obtained phase-resolved spectroscopy to confirm that all systems are AM CVn binaries, and we obtained high-speed photometry to confirm the eclipse and characterize the systems. The spectra of two long-period systems (61.5 and 53.3 minutes) show many emission and absorption lines, indicating the presence of N, O, Na, Mg, Si, and Ca, and also the K and Zn, elements which have never been detected in AM CVn systems before. By modelling the high-speed photometry, we measured the mass and radius of the donor star, potentially constraining the evolutionary channel that formed these AM CVn systems. We determined that the average mass of the accreting white dwarf is $\approx0.8$$\mathrm{M_{\odot}}$, and that the white dwarfs in long-period systems are hotter than predicted by recently updated theoretical models. The donors have a high entropy and are a factor of $\approx$ 2 more massive compared to zero-entropy donors at the same orbital period. The large donor radius is most consistent with He-star progenitors, although the observed spectral features seem to contradict this. The discovery of 5 new eclipsing AM~CVn systems is consistent with the known observed AM CVn space density and estimated ZTF recovery efficiency. Based on this estimate, we expect to find another 1--4 eclipsing AM CVn systems as ZTF continues to obtain data. This will further increase our understanding of the population, but will require high precision data to better characterize these 5 systems and any new discoveries., Comment: submitted to MNRAS

AR Scorpii (AR Sco) is the only radio-pulsing white dwarf known to date. It shows a broad-band spectrum extending from radio to X-rays whose luminosity cannot be explained by thermal emission from the system components alone, and is instead explained through synchrotron emission powered by the spin-down of the white dwarf. We analysed NTT/ULTRACAM, TNT/ULTRASPEC, and GTC/HiPERCAM high-speed photometric data for AR Sco spanning almost seven years and obtained a precise estimate of the spin frequency derivative, now confirmed with 50-sigma significance. Using archival photometry, we show that the spin down rate of P/Pdot = 5.6e6 years has remained constant since 2005. As well as employing the method of pulse-arrival time fitting used for previous estimates, we also found a consistent value via traditional Fourier analysis for the first time. In addition, we obtained optical time-resolved spectra with WHT/ISIS and VLT/X-shooter. We performed modulated Doppler tomography for the first time for the system, finding evidence of emission modulated on the orbital period. We have also estimated the projected rotational velocity of the M-dwarf as a function of orbital period and found that it must be close to Roche lobe filling. Our findings provide further constraints for modelling this unique system., 15 pages, 18 figures. Accepted for publication in MNRAS

Over the past few years, $\sim$30 extragalactic fast X-ray transients (FXRTs) have been discovered, mainly in Chandra and XMM-Newton data. Their nature remains unclear, with proposed origins including a double neutron star merger, a tidal disruption event involving an intermediate-mass black hole and a white dwarf, or a supernova shock breakout. A decisive differentiation between these three promising mechanisms for their origin requires an understanding of the FXRT energetics, environments, and/or host properties. We present optical observations obtained with the Very Large Telescope for the FXRTs XRT 000519 and XRT 110103 and Gran Telescopio Canarias observations for XRT 000519 designed to search for host galaxies of these FXRTs. In the $g_s$, $r_s$ and $R$-band images, we detect an extended source on the North-West side of the $\sim$ $1^{\prime\prime}$ (68% confidence) error circle of the X-ray position of XRT 000519 with a Kron magnitude of $g_s=$26.29$\pm$0.09 (AB magnitude). We discuss the XRT 000519 association with the probable host candidate for various possible distances, and we conclude that if XRT 000519 is associated with the host candidate a supernova shock breakout scenario is likely excluded. No host galaxy is found near XRT 110103 down to a limiting magnitude of $R>25.8$., 11 pages, 11 figures, submitted to MNRAS

Large-scale sky surveys have played a transformative role in our understanding of astrophysical transients, only made possible by increasingly powerful machine learning-based filtering to accurately sift through the vast quantities of incoming data generated. In this paper, we present a new real-bogus classifier based on a Bayesian convolutional neural network that provides nuanced, uncertainty-aware classification of transient candidates in difference imaging, and demonstrate its application to the datastream from the GOTO wide-field optical survey. Not only are candidates assigned a well-calibrated probability of being real, but also an associated confidence that can be used to prioritise human vetting efforts and inform future model optimisation via active learning. To fully realise the potential of this architecture, we present a fully-automated training set generation method which requires no human labelling, incorporating a novel data-driven augmentation method to significantly improve the recovery of faint and nuclear transient sources. We achieve competitive classification accuracy (FPR and FNR both below 1%) compared against classifiers trained with fully human-labelled datasets, whilst being significantly quicker and less labour-intensive to build. This data-driven approach is uniquely scalable to the upcoming challenges and data needs of next-generation transient surveys. We make our data generation and model training codes available to the community., 17 pages, 12 figures, resubmitted to MNRAS following reviewer comments

Over a dozen millisecond pulsars are ablating low-mass companions in close binary systems. In the original "black widow", the 8-hour orbital period eclipsing pulsar PSR J1959+2048 (PSR B1957+20), high energy emission originating from the pulsar is irradiating and may eventually destroy a low-mass companion. These systems are not only physical laboratories that reveal the dramatic result of exposing a close companion star to the relativistic energy output of a pulsar, but are also believed to harbour some of the most massive neutron stars, allowing for robust tests of the neutron star equation of state. Here, we report observations of ZTF J1406+1222, a wide hierarchical triple hosting a 62-minute orbital period black widow candidate whose optical flux varies by a factor of more than 10. ZTF J1406+1222 pushes the boundaries of evolutionary models, falling below the 80 minute minimum orbital period of hydrogen-rich systems. The wide tertiary companion is a rare low metallicity cool subdwarf star, and the system has a Galactic halo orbit consistent with passing near the Galactic center, making it a probe of formation channels, neutron star kick physics, and binary evolution., 63 pages, 15 figures, 2 tables, published in Nature on May 4, 2022

We present the discovery of only the third brown dwarf known to eclipse a non-accreting white dwarf. Gaia parallax information and multi-colour photometry confirm that the white dwarf is cool (9950$\pm$150K) and has a low mass (0.45$\pm$0.05~MSun), and spectra and lightcurves suggest the brown dwarf has a mass of 0.067 $\pm$0.006 MSun (70 MJup) and a spectral type of L5 $\pm$1. The kinematics of the system show that the binary is likely to be a member of the thick disk and therefore at least 5 Gyr old. The high cadence lightcurves show that the brown dwarf is inflated, making it the first brown dwarf in an eclipsing white dwarf-brown dwarf binary to be so., Accepted for publication in MNRAS, 11 pages, 7 Figures

Of more than a thousand known cataclysmic variables (CVs), where a white dwarf is accreting from a hydrogen-rich star, only a dozen have orbital periods below 75 minutes

Using photometric ULTRACAM observations of three new short period cataclysmic variables, we model the primary eclipse lightcurves to extract the orbital separation, masses, and radii of their component stars. We find donor masses of 0.060 +/- 0.008 solar masses, 0.042 +/- 0.001 solar masses, and 0.042 +/- 0.004 solar masses, two being very low-mass sub-stellar donors, and one within 2 sigma of the hydrogen burning limit. All three of the new systems lie close to the modified, "optimal" model evolutionary sequence of Knigge et al. (2011). We briefly re-evaluate the long-standing discrepancy between observed donor mass and radius data, and theoretical CV evolutionary tracks. By looking at the difference in the observed period at each mass and the period predicted by the Knigge et al. (2011) evolutionary sequence, we qualitatively examine the form of excess angular momentum loss that is missing from the models below the period gap. We show indications that the excess angular momentum loss missing from CV models grows in importance relative to gravitational losses as the period decreases. Detailed CV evolutionary models are necessary to draw more quantitative conclusions in the future., 17 pages, 14 figures, 5 tables

We present an $i$-band photometric study of over 800 young stellar objects in the OB association Cep OB3b, which samples timescales from 1 minute to ten years. Using structure functions we show that on all timescales ($\tau$) there is a monotonic decrease in variability from Class I to Class II through the transition disc (TD) systems to Class III, i.e. the more evolved systems are less variable. The Class Is show an approximately power-law increase ($\tau^{0.8}$) in variability from timescales of a few minutes to ten years. The Class II, TDs and Class III systems show a qualitatively different behaviour with most showing a power-law increase in variability up to a timescale corresponding to the rotational period of the star, with little additional variability beyond that timescale. However, about a third of the Class IIs show lower overall variability, but their variability is still increasing at 10 years. This behaviour can be explained if all Class IIs have two primary components to their variability. The first is an underlying roughly power-law variability spectrum, which evidence from the infrared suggests is driven by accretion rate changes. The second component is approximately sinusoidal and results from the rotation of the star. We suggest that the systems with dominant longer-timescale variability have a smaller rotational modulation either because they are seen at low inclinations or have more complex magnetic field geometries. We derive a new way of calculating structure functions for large simulated datasets (the "fast structure function"), based on fast Fourier transforms., Comment: 23 pages and 22 figures. Accepted for publication in MNRAS. At time of submission the electronic tables are only available from https:// doi.org/10.24378/exe.2124

The typical detection rate of $\sim1$ gamma-ray burst (GRB) per day by the \emph{Fermi} Gamma-ray Burst Monitor (GBM) provides a valuable opportunity to further our understanding of GRB physics. However, the large uncertainty of the \emph{Fermi} localization typically prevents rapid identification of multi-wavelength counterparts. We report the follow-up of 93 \emph{Fermi} GRBs with the Gravitational-wave Optical Transient Observer (GOTO) prototype on La Palma. We selected 53 events (based on favourable observing conditions) for detailed analysis, and to demonstrate our strategy of searching for optical counterparts. We apply a filtering process consisting of both automated and manual steps to 60\,085 candidates initially, rejecting all but 29, arising from 15 events. With $\approx3$ GRB afterglows expected to be detectable with GOTO from our sample, most of the candidates are unlikely to be related to the GRBs. Since we did not have multiple observations for those candidates, we cannot confidently confirm the association between the transients and the GRBs. Our results show that GOTO can effectively search for GRB optical counterparts thanks to its large field of view of $\approx40$ square degrees and its depth of $\approx20$ mag. We also detail several methods to improve our overall performance for future follow-up programs of \emph{Fermi} GRBs.

The advent of wide-field sky surveys has led to the growth of transient and variable source discoveries. The data deluge produced by these surveys has necessitated the use of machine learning (ML) and deep learning (DL) algorithms to sift through the vast incoming data stream. A problem that arises in real-world applications of learning algorithms for classification is imbalanced data, where a class of objects within the data is underrepresented, leading to a bias for over-represented classes in the ML and DL classifiers. We present a recurrent neural network (RNN) classifier that takes in photometric time-series data and additional contextual information (such as distance to nearby galaxies and on-sky position) to produce real-time classification of objects observed by the Gravitational-wave Optical Transient Observer (GOTO), and use an algorithm-level approach for handling imbalance with a focal loss function. The classifier is able to achieve an Area Under the Curve (AUC) score of 0.972 when using all available photometric observations to classify variable stars, supernovae, and active galactic nuclei. The RNN architecture allows us to classify incomplete light curves, and measure how performance improves as more observations are included. We also investigate the role that contextual information plays in producing reliable object classification., Comment: 16 pages, 12 figures, to be published in Monthly Notices of the Royal Astronomical Society

We present optical photometry of the cataclysmic variable LAMOST J024048.51+195226.9 taken with the high-speed, five-band CCD camera HiPERCAM on the 10.4 m Gran Telescopio Canarias (GTC). We detect pulsations originating from the spin of its white dwarf, finding a spin period of 24.9328(38)s. The pulse amplitude is of the order of 0.2% in the g-band, below the detection limits of previous searches. This detection establishes LAMOST J024048.51+195226.9 as only the second white dwarf magnetic propeller system, a twin of its long-known predecessor, AE Aquarii. At 24.93s, the white dwarf in LAMOST J024048.51+195226.9 has the shortest known spin period of any cataclysmic variable star. The white dwarf must have a mass of at least 0.7MSun to sustain so short a period. The observed faintest u-band magnitude sets an upper limit on the white dwarf's temperature of ~25000K. The pulsation amplitudes measured in the five HiPERCAM filters are consistent with an accretion spot of ~30000K covering ~2% of the white dwarf's visible area, although much hotter and smaller spots cannot be ruled out.

Full list of authors: Dhillon, V. S.; Bezawada, N.; Black, M.; Dixon, S. D.; Gamble, T.; Gao, X.; Henry, D. M.; Kerry, P.; Littlefair, S. P.; Lunney, D. W.; Marsh, T. R.; Miller, C.; Parsons, S. G.; Ashley, R. P.; Breedt, E.; Brown, A.; Dyer, M. J.; Green, M. J.; Pelisoli, I.; Sahman, D. I. Wild, J.; Ives, D. J.; Mehrgan, L.; Stegmeier, J.; Dubbeldam, C. M.; Morris, T. J.; Osborn, J.; Wilson, R. W.; Casares, J.; Muñoz-Darias, T.; Pallé, E.; Rodríguez-Gil, P.; Shahbaz, T.; Torres, M. A. P.; de Ugarte Postigo, A.; Cabrera-Lavers, A.; Corradi, R. L. M.; Domínguez, R. D.; García-Alvarez, D., HiPERCAM is a portable, quintuple-beam optical imager that saw first light on the 10.4-m Gran Telescopio Canarias (GTC) in 2018. The instrument uses re-imaging optics and four dichroic beamsplitters to record us, gs, rs, is, zs (320-1060 nm) images simultaneously on its five CCD cameras, each of 3.1-arcmin (diagonal) field of view. The detectors in HiPERCAM are frame-transfer devices cooled thermo-electrically to 183 K, thereby allowing both long-exposure, deep imaging of faint targets, as well as high-speed (over 1000 windowed frames per second) imaging of rapidly varying targets. A comparison-star pick-off system in the telescope focal plane increases the effective field of view to 6.7 arcmin for differential photometry. Combining HiPERCAM with the world's largest optical telescope enables the detection of astronomical sources to gs ∼23 in 1 s and gs ∼28 in 1 h. In this paper, we describe the scientific motivation behind HiPERCAM, present its design, report on its measured performance, and outline some planned enhancements. © 2021 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society., HiPERCAM was funded by the European Research Council under the European Union's Seventh Framework Programme (FP/20072013) under ERC-2013-ADG GrantAgreement no. 340040 (HiPERCAM), with additional funding for operations and enhancements provided by the Science and Technology Facilities Council (STFC). This paper is based on observations made with the Gran Telescopio Canarias, installed at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, on the island of La Palma. SGP acknowledges the support of an STFC Ernest Rutherford Fellowship. PR-G and TMD acknowledge support from the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU), and the European Regional Development Fund (FEDER) under grant AYA2017-83383-P. MAPT and TM-D acknowledge support via Ram ' on y Cajal Fellowships RYC-2015-17854 and RYC-201518148., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

Full list of authors: Hodgkin, S. T.; Harrison, D. L.; Breedt, E.; Wevers, T.; Rixon, G.; Delgado, A.; Yoldas, A.; Kostrzewa-Rutkowska, Z.; Wyrzykowski, Ł.; van Leeuwen, M.; Blagorodnova, N.; Campbell, H.; Eappachen, D.; Fraser, M.; Ihanec, N.; Koposov, S. E.; Kruszyńska, K.; Marton, G.; Rybicki, K. A.; Brown, A. G. A.; Burgess, P. W.; Busso, G.; Cowell, S.; De Angeli, F.; Diener, C.; Evans, D. W.; Gilmore, G.; Holland, G.; Jonker, P. G.; van Leeuwen, F.; Mignard, F.; Osborne, P. J.; Portell, J.; Prusti, T.; Richards, P. J.; Riello, M.; Seabroke, G. M.; Walton, N. A.; Ábrahám, P.; Altavilla, G.; Baker, S. G.; Bastian, U.; O'Brien, P.; de Bruijne, J.; Butterley, T.; Carrasco, J. M.; Castañeda, J.; Clark, J. S.; Clementini, G.; Copperwheat, C. M.; Cropper, M.; Damljanovic, G.; Davidson, M.; Davis, C. J.; Dennefeld, M.; Dhillon, V. S.; Dolding, C.; Dominik, M.; Esquej, P.; Eyer, L.; Fabricius, C.; Fridman, M.; Froebrich, D.; Garralda, N.; Gomboc, A.; González-Vidal, J. J.; Guerra, R.; Hambly, N. C.; Hardy, L. K.; Holl, B.; Hourihane, A.; Japelj, J.; Kann, D. A.; Kiss, C.; Knigge, C.; Kolb, U.; Komossa, S.; Kóspál, Á.; Kovács, G.; Kun, M.; Leto, G.; Lewis, F.; Littlefair, S. P.; Mahabal, A. A.; Mundell, C. G.; Nagy, Z.; Padeletti, D.; Palaversa, L.; Pigulski, A.; Pretorius, M. L.; van Reeven, W.; Ribeiro, V. A. R. M.; Roelens, M.; Rowell, N.; Schartel, N.; Scholz, A.; Schwope, A.; Sipőcz, B. M.; Smartt, S. J.; Smith, M. D.; Serraller, I.; Steeghs, D.; Sullivan, M.; Szabados, L.; Szegedi-Elek, E.; Tisserand, P.; Tomasella, L.; van Velzen, S.; Whitelock, P. A.; Wilson, R. W.; Young, D. R.-- This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited., Context. Since July 2014, the Gaia mission has been engaged in a high-spatial-resolution, time-resolved, precise, accurate astrometric, and photometric survey of the entire sky. Aims. We present the Gaia Science Alerts project, which has been in operation since 1 June 2016. We describe the system which has been developed to enable the discovery and publication of transient photometric events as seen by Gaia. Methods. We outline the data handling, timings, and performances, and we describe the transient detection algorithms and filtering procedures needed to manage the high false alarm rate. We identify two classes of events: (1) sources which are new to Gaia and (2) Gaia sources which have undergone a significant brightening or fading. Validation of the Gaia transit astrometry and photometry was performed, followed by testing of the source environment to minimise contamination from Solar System objects, bright stars, and fainter near-neighbours. Results. We show that the Gaia Science Alerts project suffers from very low contamination, that is there are very few false-positives. We find that the external completeness for supernovae, CE = 0.46, is dominated by the Gaia scanning law and the requirement of detections from both fields-of-view. Where we have two or more scans the internal completeness is CI = 0.79 at 3 arcsec or larger from the centres of galaxies, but it drops closer in, especially within 1 arcsec. Conclusions. The per-Transit photometry for Gaia transients is precise to 1% at G = 13, and 3% at G = 19. The per-Transit astrometry is accurate to 55 mas when compared to Gaia DR2. The Gaia Science Alerts project is one of the most homogeneous and productive transient surveys in operation, and it is the only survey which covers the whole sky at high spatial resolution (subarcsecond), including the Galactic plane and bulge. © S. T. Hodgkin et al. 2021., This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. Further details of funding authorities and individuals contributing to the success of the mission is shown at https://gea.esac.esa.int/archive/documentation/GEDR3/Miscellaneous/sec_acknowl/. We thank the United Kingdom Particle Physics and Astronomy Research Council (PPARC), the United Kingdom Science and Technology Facilities Council (STFC), and the United Kingdom Space Agency (UKSA) through the following grants to the University of Bristol, the University of Cambridge, the University of Edinburgh, the University of Leicester, the Mullard Space Sciences Laboratory of University College London, and the United Kingdom Rutherford Appleton Laboratory (RAL): PP/D006511/1, PP/D006546/1, PP/D006570/1, ST/I000852/1, ST/J005045/1, ST/K00056X/1, ST/K000209/1, ST/K000756/1, ST/L006561/1, ST/N000595/1, ST/S000623/1, ST/N000641/1, ST/N000978/1, ST/N001117/1, ST/S000089/1, ST/S000976/1, ST/S001123/1, ST/S001948/1, ST/S002103/1, and ST/V000969/1. This paper made use of the Whole Sky Database (WSDB) created by Sergey Koposov and maintained at the Institute of Astronomy, Cambridge with financial support from the Science and Technology Facilities Council (STFC) and the European Research Council (ERC). We thank the William Herschel and Isaac Newton Telescopes on the Roque de los Muchachos Observatory, La Palma, Spain, as well as the Optical Infrared Coordination Network for Astronomy (OPTICON) for their support of this project through telescope time, especially during the commissioning and verification phases. We thank the Copernico 1.82 m telescope (Mt. Ekar, Asiago Italy) operated by INAF Padova for supporting the project through telescope time (under the Large Programme Tomasella-SNe) during the verification phases. We acknowledge observations taken as part of the PESSTO project collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 199.D-0143. Authors at the ICCUB were supported by the Spanish Ministry of Science, Innovation and University (MICIU/FEDER, UE) through grant RTI2018-095076-B-C21, and the Institute of Cosmos Sciences University of Barcelona (ICCUB, Unidad de Excelencia ’María de Maeztu’) through grant CEX2019-000918-M. This work is supported by Polish NCN grants: Daina No. 2017/27/L/ST9/03221, Harmonia No. 2018/30/M/ST9/00311, Preludium No. 2017/25/N/ST9/01253 and MNiSW grant DIR/WK/2018/12 as well as the European Commission’s Horizon2020 OPTICON grant No. 730890. The Authors would like to thank the Warsaw University OGLE project for their continuous support in this work. AB acknowledges financial support from the Netherlands Research School for Astronomy (NOVA). AG acknowledges the financial support from the Slovenian Research Agency (grants P1-0031, I0-0033, J1-8136, J1-2460). AH was funded in part by the Leverhulme Trust through grant RPG-2012-541 and by the European Research Council grant 320360. AP acknowledges support from the NCN grant no. 2016/21/B/ST9/01126. CM acknowledges support from Jim and Hiroko Sherwin. DAK acknowledges support from the Spanish research projects AYA 2014-58381-P, AYA2017-89384-P, from Juan de la Cierva Incorporación fellowship IJCI-2015-26153, and from Spanish National Research Project RTI2018-098104-J-I00 (GRBPhot). EB and STH are funded by the Science and Technology Facilities Council grant ST/S000623/1. TW was funded in part by European Research Council grant 320360 and by European Commission grant 730980. GC acknowledges the Agenzia Spaziale Italiana (ASI) for its continuing support through contract 2018-24-HH.0 to the Italian Istituto Nazionale di Astrofisica (INAF). GD acknowledges the observing grant support from the Institute of Astronomy and Rozhen NAO BAS through the bilateral joint research project “Gaia Celestial Reference Frame (CRF) and fast variable astronomical objects” (during 2020-2022, leader is G. Damljanovic), and support by the Ministry of Education, Science and Technological Development of the Republic of Serbia (contract no. 451-03-68/2020-14/200002). G. Marton acknowledges support from the EC Horizon 2020 project OPTICON (730890) and the ESA PRODEX contract no. 4000129910. MF is supported by a Royal Society - Science Foundation Ireland University Research Fellowship NB acknowledges support from the research programme VENI, with project number 016.192.277, which is (partly) financed by the Netherlands Organisation for Scientific Research (NWO). NI is partially supported by Polish NCN DAINA grant no. 2017/27/L/ST9/03221. PAW acknowledges research funding from the South African National Research Foundation. RWW was funded by the Science and Technology Facilities Council grant ST/P000541/1. V.A.R.M.R. acknowledges financial support from Radboud Excellence Initiative, the Fundação para a Ciência e a Tecnologia (FCT) in the form of an exploratory project of reference IF/00498/2015/CP1302/CT0001, FCT and the Ministério da Ciência, Tecnologia e Ensino Superior (MCTES) through national funds and when applicable co-funded EU funds under the project UIDB/EEA/50008/2020, and supported by Enabling Green E-science for the Square Kilometre Array Research Infrastructure (ENGAGE-SKA), POCI-01-0145-FEDER-022217, and PHOBOS, POCI-01-0145-FEDER-029932, funded by Programa Operacional Competitividade e Internacionalização (COMPETE 2020) and FCT, Portugal. ZKR acknowledges funding from the Netherlands Research School for Astronomy (NOVA). ZN acknowledges support from the ESA PRODEX contract nr. 4000129910., With funding from the Spanish government through the Severo Ochoa Centre of Excellence accreditation SEV-2017-0709.

We report on a multi-epoch campaign of rapid optical/X-ray timing observations of the superbright 2018 outburst of MAXI J1820+070, a black hole low-mass X-ray binary system. The observations spanned 80 days in the initial hard-state, and were taken with NTT/ULTRACAM and GTC/HiPERCAM in the optical (ugriz filters at time resolutions of 8--300 Hz) and with ISS/NICER in X-rays. We find (i) a growing anti-correlation between the optical and X-ray lightcurves, (ii) a steady, positive correlation at an optical lag of 0.2 s (with a longer lag at longer wavelengths) present in all epochs, and (iii) a curious positive correlation at \textit{negative} optical lags in the last, X-ray softest epoch, with longer wavelengths showing a greater correlation and a more negative lag. To explain these we postulate the possible existence of two synchrotron-emitting components; a compact jet and a hot flow. In our model, the significance of the jet decreases over the outburst, while the hot flow remains static (thus, relatively, increasing in significance). We also discuss a previously discovered quasi-periodic oscillation and note how it creates coherent optical time lags, stronger at longer wavelengths, during at least two epochs., Comment: 19 pages, 13 figures. Accepted for publication to MNRAS

Magnitude-limited samples have shown that 20-25 per cent of cataclysmic variables contain white dwarfs with magnetic fields of Mega Gauss strength, in stark contrast to the approximately 5 per cent of single white dwarfs with similar magnetic field strengths. Moreover, the lack of identifiable progenitor systems for magnetic cataclysmic variables leads to considerable challenges when trying to understand how these systems form and evolve. Here we present a sample of six magnetic white dwarfs in detached binaries with low-mass stellar companions where we have constrained the stellar and binary parameters including, for the first time, reliable mass estimates for these magnetic white dwarfs. We find that they are systematically more massive than non-magnetic white dwarfs in detached binaries. These magnetic white dwarfs generally have cooling ages of more than 1 Gyr and reside in systems that are very close to Roche-lobe filling. Our findings are more consistent with these systems being temporarily detached cataclysmic variables, rather than pre-cataclysmic binaries, but we cannot rule out the latter possibility. We find that these systems can display unusual asymmetric light curves that may offer a way to identify them in larger numbers in future. Seven new candidate magnetic white dwarf systems are also presented, three of which have asymmetric light curves. Finally, we note that several newly identified magnetic systems have archival spectra where there is no clear evidence of magnetism, meaning that these binaries have been previously missed. Nevertheless, there remains a clear lack of younger detached magnetic white dwarf systems., Comment: Accepted for publication in MNRAS

We present a detailed study of the stellar and orbital parameters of the post-common envelope binary central star of the planetary nebula Ou~5. Low-resolution spectra obtained during the primary eclipse -- to our knowledge the first isolated spectra of the companion to a post-common-envelope planetary nebula central star -- were compared to catalogue spectra, indicating that the companion star is a late K- or early M-type dwarf. Simultaneous modelling of multi-band photometry and time-resolved radial velocity measurements was then used to independently determine the parameters of both stars as well as the orbital period and inclination. The modelling indicates that the companion star is low mass ($\sim$0.25~M$_\odot$) and has a radius significantly larger than would be expected for its mass. Furthermore, the effective temperature and surface gravity of nebular progenitor, as derived by the modelling, do not lie on single-star post-AGB evolutionary tracks, instead being more consistent with a post-RGB evolution. However, an accurate determination of the component masses is challenging. This is principally due to the uncertainty on the locus of the spectral lines generated by the irradiation of the companion's atmosphere by the hot primary (used to derive companion star's radial velocities), as well as the lack of radial velocities of the primary., Comment: 9 pages, 6 figures, MNRAS

The Gravitational-wave Optical Transient Observer (GOTO) is a wide-field telescope project focused on detecting optical counterparts to gravitational-wave sources. The GOTO Telescope Control System (G-TeCS) is a custom robotic control system which autonomously manages the GOTO telescope hardware and nightly operations. Since the commissioning the GOTO prototype on La Palma in 2017, development of the control system has focused on the alert handling and scheduling systems. These allow GOTO to receive and process transient alerts and then schedule and carry out observations, all without the need for human involvement. GOTO is ultimately anticipated to include multiple telescope arrays on independent mounts, both on La Palma and at a southern site in Australia. When complete these mounts will be linked to form a single multi-site observatory, requiring more advanced scheduling systems to best optimise survey and follow-up observations., 12 pages, 6 figures, 2 tables, submitted to SPIE Astronomical Telescopes + Instrumentation 2020

Using photometry collected with the Zwicky Transient Facility (ZTF), we are conducting an ongoing survey for binary systems with short orbital periods ($P_{\rm b}, 32 pages, 12 figures, 7 tables, submitted to ApJ

Subdwarf B stars are core-helium burning stars located on the extreme horizontal branch. Extensive mass loss on the red giant branch is necessary to form them. It has been proposed that substellar companions could lead to the required mass-loss when they are engulfed in the envelope of the red giant star. J08205+0008 was the first example of a hot subdwarf star with a close, substellar companion candidate to be found. Here we perform an in-depth re-analysis of this important system with much higher quality data allowing additional analysis methods. From the higher resolution spectra obtained with ESO-VLT/XSHOOTER we derive the chemical abundances of the hot subdwarf as well as its rotational velocity. Using the { it Gaia} parallax and a fit to the spectral energy distribution in the secondary eclipse, tight constraints to the radius of the hot subdwarf are derived. From a long-term photometric campaign we detected a significant period decrease of $-3.2(8)\cdot 10^{-12} \,\rm dd^{-1}$. This can be explained by the non-synchronised hot subdwarf star being spun up by tidal interactions forcing it to become synchronised. From the rate of period decrease we could derive the synchronisation timescale to be 4 Myr, much smaller than the lifetime on EHB. By combining all different methods we could constrain the hot subdwarf to a mass of $0.39-0.50\,\rm M_\odot$ and a radius of $R_{\rm sdB}=0.194\pm0.008\,\rm R_\odot$, and the companion to $0.061-0.071\rm\,M_\odot$ with a radius of $R_{\rm comp}=0.092 \pm 0.005\,\rm R_\odot$, below the hydrogen burning limit. We therefore confirm that the companion is most likely a massive brown dwarf., accepted to MNRAS, 25 pages

Neptune’s atmosphere is highly dynamic with atmospheric systems observable as bands and discrete cloud systems that evolve in time scales of days, weeks and years. Most of them are observed as tropospheric clouds and elevated hazes that appear highly contrasted in observations obtained in hydrogen and methane absorption bands in the red and near-infrared spectrum of the planet. Given the small size of Neptune as observed from Earth (2.3 arcsec), it is difficult to characterize most of these clouds. Basic questions such as if they are convective storms, vortices or clouds detached from atmospheric waves or bands can be difficult for an specific feature in a given observation [1]. Only Adaptive Optics or lucky-imaging instruments in 8-m telescopes or larger, and HST, can provide suitable data, but the difficulty to access enough observational time in these facilities suggests that a combination of data from several observing programs can help. Smaller telescopes can also play an important role since they can be used to follow the main cloud systems and cover the gaps between observations obtained by the larger telescopes. This can provide the life-time or drift rates of the largest meteorological systems allowing to compare observations of the same features observed months apart in the largest telescopes. During the last few years we have combined observations obtained from a variety of telescopes to study the major cloud systems and understand their life-time and evolution [2, 3], including those of “companion” clouds linked to rare dark vortices that are only observable in blue wavelengths from space [2, 4, 5]. In this work we present our data for 2019 which consists of the following observations: HST observations from the Outer Planets Atmospheres Legacy program (OPAL). Several sets from Keck and Lick telescopes from different programs including some relatively frequent observations from the TWILIGHT program. GTC observations with the HiperCam instrument doing lucky-imaging. Calar Alto 2.2m telescope with the PlanetCam lucky-imaging instrument. One single observation from Gemini while testing an AO system. Additional observations from the Pic du Midi 1.05 m telescope. Images provided by amateur astronomers and available through the PVOL [6] database. The combination of these data suggests more variability and less cloud activity in 2019 than in previous years with a lower number of features in the data sets obtained with smaller telescopes. We provide the identification of particular meteorological systems over late summer 2019 and present drift rates of different mid-latitude features in the south hemisphere that are close but separated enough to the Voyager zonal winds to deserve attention. Other cloud systems in the south polar region and north tropics seem to follow the Voyager wind profile. Future punctual observations achievable with new observational facilities such as the JWST will benefit from the evolutionary time-lines of the major cloud systems of Neptune and their drift rates in the atmosphere provided by similar future campaigns. References [1] Hueso and Sánchez-Lavega, Atmospheric Dynamics and Vertical Structure of Uranus and Neptune's weather layers. Space Science Reviews, 2019. [2] Hueso et al., Neptune long-lived atmospheric features in 2013-2015 from small (28-cm) to large (10-m) telescopes. Icarus, 2017. [3] Molter et al., Analysis of Neptune's 2017 Bright Equatorial Storm, Icarus, 2019. [4] Wong et al., A New Dark vortex on Neptune, The Astronomical Journal, 2018. [5] Hsu et al., Lifetimes and Occurrence Rates of Dark Vortices on Neptune from 25 Years of Hubble Space Telescope Images, The Astronomical Journal, 2018. [6] Hueso et al., The Planetary Virtual Observatory and Laboratory (PVOL) and its integration into the Virtual European Solar and Planetary Access (VESPA), Planetary Space Science, 2018.

White dwarfs are the burnt out cores of Sun-like stars and are the final fate of 97% of all stars in our Galaxy. The internal structure and composition of white dwarfs are hidden by their high gravities, which causes all elements, apart from the lightest ones, to settle out of their atmospheres. The most direct method to probe the inner structure of stars and white dwarfs in detail is via asteroseismology. Here we present the first known pulsating white dwarf in an eclipsing binary system, enabling us to place extremely precise constraints on the mass and radius of the white dwarf from the light curve, independent of the pulsations. This 0.325M$_\odot$ white dwarf --- one member of SDSS J115219.99+024814.4 --- will serve as a powerful benchmark to constrain empirically the core composition of low-mass stellar remnants and investigate the effects of close binary evolution on the internal structure of white dwarfs., Accepted for publication in Nature Astronomy

The amount of observational data produced by time-domain astronomy is exponentially in-creasing. Human inspection alone is not an effective way to identify genuine transients fromthe data. An automatic real-bogus classifier is needed and machine learning techniques are commonly used to achieve this goal. Building a training set with a sufficiently large number of verified transients is challenging, due to the requirement of human verification. We presentan approach for creating a training set by using all detections in the science images to be thesample of real detections and all detections in the difference images, which are generated by the process of difference imaging to detect transients, to be the samples of bogus detections. This strategy effectively minimizes the labour involved in the data labelling for supervised machine learning methods. We demonstrate the utility of the training set by using it to train several classifiers utilizing as the feature representation the normalized pixel values in 21-by-21pixel stamps centered at the detection position, observed with the Gravitational-wave Optical Transient Observer (GOTO) prototype. The real-bogus classifier trained with this strategy can provide up to 95% prediction accuracy on the real detections at a false alarm rate of 1%., 9 pages, 8 figures

Full author list: E. Szegedi-Elek, P. Ábrahám, Ł. Wyrzykowski, M. Kun, Á. Kóspál, L. Chen, G. Marton, A. Moór, C. Kiss, A. Pál, L. Szabados, J. Varga, E. Varga-Verebélyi, C. Andreas, E. Bachelet, R. Bischoff, A. Bódi, E. Breedt, U. Burgaz, T. Butterley, J. M. Carrasco, V. Čepas, G. Damljanovic, I. Gezer, V. Godunova, M. Gromadzki, A. Gurgul, L. Hardy, F. Hildebrandt, S. Hoffmann, M. Hundertmark, N. Ihanec, R. Janulis, Cs. Kalup, Z. Kaczmarek, R. Könyves-Tóth, M. Krezinger, K. Kruszyńska, S. Littlefair, M. Maskoliūnas, L. Mészáros, P. Mikołajczyk, M. Mugrauer, H. Netzel, A. Ordasi, E. Pakštienė, K. A. Rybicki, K. Sárneczky, B. Seli, A. Simon, K. Šiškauskaitė, Á. Sódor, K. V. Sokolovsky2, W. Stenglein, R. Street, R. Szakáts, L. Tomasella, Y. Tsapras, K. Vida, J. Zdanavičius, M. Zieliński, P. Zieliński, and O. Ziółkowska, We present optical-infrared photometric and spectroscopic observations of Gaia 18dvy, located in the Cygnus OB3 association at a distance of 1.88 kpc. Gaia 18dvy was noted by the Gaia alerts system when its light curve exhibited a ⪆4 mag rise in 2018-2019. The brightening was also observable at mid-infared wavelengths. The infrared colors of Gaia 18dvy became bluer as the outburst progressed. Its optical and near-infrared spectroscopic characteristics in the outburst phase are consistent with those of bona fide FU Orionis-type young eruptive stars. The progenitor of the outburst is probably a low-mass K-type star with an optical extinction of ∼3 mag. A radiative transfer modeling of the circumstellar structure, based on the quiescent spectral energy distribution, indicates a disk with a mass of 4 10-3 M o˙. Our simple accretion disk modeling implies that the accretion rate had been exponentially increasing for more than 3 yr until mid-2019, when it reached a peak value of 6.9 10-6 M o˙ yr-1. In many respects, Gaia 18dvy is similar to the FU Ori-type object HBC 722., We thank the anonymous referee, whose questions and comments significantly improved the paper. This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program under grant agreement No. 716155 (SACCRED), Lendület LP2018-7/2019 and KEP-7/2018 of the Hungarian Academy of Sciences, GINOP 2.3.2-15-2016-00003 and PD-128360 of the Hungarian National Research, Development and Innovation Office, Polish NCN DAINA grant 2017/27/ L/ST9/03221, European Commission’s Horizon 2020 OPTICON grant 730890, Polish MNiSW grant DIR/WK/2018/12, grant No. S-LL-19-2 of the Research Council of Lithuania, Project No. 176011 “Dynamics and kinematics of celestial bodies and systems” of the Ministry of Education, Science and Technological Development of the Republic of Serbia, DFG priority program SPP 1992 “Exploring the Diversity of Extrasolar Planets” (WA 1047/11-1), the MINECO (Spanish Ministry of Economy) through grant RTI2018-095076-B-C21 (MINECO/FEDER, UE). The Joan Oró Telescope (TJO) of the Montsec Astronomical Observatory (OAdM) is owned by the Catalan Government and is operated by the Institute for Space Studies of Catalonia (IEEC). MG is supported by the Polish NCN MAESTRO grant 2014/14/ A/ST9/00121. We acknowledge ESA Gaia, DPAC, and the Photometric Science Alerts Team. We thank Christina Conner, Megan Davis, Alessandro Dellarovere, Hannah Gallamore, Mira Ghazali, Aaron Kruskie, Dylan Mankel, Jesse Leahy–McGregor, Brandon McIntyre, Barrett Ross, Courtney Wicklund, and Evan Zobel for observing Gaia 18dvy at the Michigan State University Observatory. Based on observations made with the Nordic Optical Telescope, operated by the Nordic Optical Telescope Scientific Association at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofísica de Canarias. Based on observations obtained with telescopes of the University Observatory Jena, which is operated by the Astrophysical Institute of the Friedrich-Schiller-University.

We report the results of optical follow-up observations of 29 gravitational-wave triggers during the first half of the LIGO-Virgo Collaboration (LVC) O3 run with the Gravitational-wave Optical Transient Observer (GOTO) in its prototype 4-telescope configuration (GOTO-4). While no viable electromagnetic counterpart candidate was identified, we estimate our 3D (volumetric) coverage using test light curves of on- and off-axis gamma-ray bursts and kilonovae. In cases where the source region was observable immediately, GOTO-4 was able to respond to a GW alert in less than a minute. The average time of first observation was $8.79$ hours after receiving an alert ($9.90$ hours after trigger). A mean of $732.3$ square degrees were tiled per event, representing on average $45.3$ per cent of the LVC probability map, or $70.3$ per cent of the observable probability. This coverage will further improve as the facility scales up alongside the localisation performance of the evolving gravitational-wave detector network. Even in its 4-telescope prototype configuration, GOTO is capable of detecting AT2017gfo-like kilonovae beyond 200~Mpc in favourable observing conditions. We cannot currently place meaningful electromagnetic limits on the population of distant ($\hat{D}_L = 1.3$~Gpc) binary black hole mergers because our test models are too faint to recover at this distance. However, as GOTO is upgraded towards its full 32-telescope, 2 node (La Palma \& Australia) configuration, it is expected to be sufficiently sensitive to cover the predicted O4 binary neutron star merger volume, and will be able to respond to both northern and southern triggers., 15 pages, 7 figures, 3 tables. Accepted for publication in MNRAS. Author's final submitted version

Following our tentative discovery of a faint shell around V1315 Aql reported in Sahman et al. (2015), we undertook deep Halpha imaging and intermediate-resolution spectroscopy of the shell. We find that the shell has its geometric centre located on V1315 Aql. The mass, spectral features and density of the shell are consistent with other nova shells, rather than planetary nebulae or supernova remnants. The radial velocity of the shell is consistent with the systemic velocity of V1315 Aql. We believe this evidence strongly suggests that the shell originates from an earlier nova event. This is the first nova shell discovered around a novalike, and supports the theory of nova-induced cycles in mass transfer rates (hibernation theory) first proposed by Shara et al. (1986)., Comment: Accepted fro publication in MNRAS. 10 pages, 7 figures

Conditions in a black hole outburst The binary system V404 Cygni consists of a red giant star orbiting a black hole. In 2015, a surge of accretion by the black hole caused the surrounding plasma to brighten suddenly for the first time since 1989, briefly becoming the brightest x-ray source in the sky. Dallilar et al. combined observations from radio, infrared, optical, and x-ray telescopes taken during the outburst. They compared how fast the flux decayed at each wavelength, which allowed them to constrain the size of the emitting region, determine that the plasma within it cooled through synchrotron radiation, and measure the magnetic field around the black hole. Science , this issue p. 1299

This article is dedicated to the memory of H.-J. Bode, J. G. Greenhill and O. Faragó for their long-standing support and participation to occultation campaigns. The work leading to these results has received funding from the European Research Council under the European Community’s H2020 2014-2020 ERC Grant Agreement n° 669416 “Lucky Star”. E.M. thanks support from Concytec-Fondecyt-PE and GA, FC-UNI for providing support during the 2012 July 18 occultation. B.S. thanks S. Para for partly supporting this research though a donation, J. P. Beaulieu for helping us accessing to the Hobart Observatory facilities and B. Warner, B. L. Gary, C. Erickson, H. Reitsema, L. Albert, P. J. Merritt, T. Hall, W. J. Romanishin, Y. J. Choi for providing data during the 2007 March 18 occultation. M.A. thanks CNPq (Grants 427700/2018-3, 310683/2017-3 and 473002/2013-2) and FAPERJ (Grant E-26/111.488/2013). J.L.O. thanks support from grant AYA2017-89637-R. P.S.S. acknowledges financial support from the European Union’s Horizon 2020 Research and Innovation Programme, under Grant Agreement no 687378, as part of the project “Small Bodies Near and Far” (SBNAF). J.L.O., R.D., P.S.S. and N.M. acknowledge financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). F.B.R. acknowledges CNPq support process 309578/2017-5. G.B.R. thanks support from the grant CAPES-FAPERJ/PAPDRJ (E26/203.173/2016). J.I.B.C. acknowledges CNPq grant 308150/2016-3. R.V.M. thanks the grants: CNPq-304544/2017-5, 401903/2016-8, and Faperj: PAPDRJ-45/2013 and E-26/203.026/2015. B.M. thanks the CAPES/Cofecub-394/2016-05 grant and CAPES/Brazil – Finance Code 001. B.M. and A.R.G.J. were financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) – Finance Code 001. TRAPPIST-North is a project funded by the University of Liège, in collaboration with Cadi Ayyad University of Marrakech (Morocco). TRAPPIST-South is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant FRFC 2.5.594.09.F, with the participation of the Swiss National Science Foundation (FNS/SNSF). VSD, SPL, TRM and ULTRACAM are all supported by the STFC. K.G. acknowledges help from the team of Archenhold-Observatory, Berlin, and A.R. thanks G. Román (Granada) for help during the observation of the 2016 July 19 occultation. A.J.C.T. acknowledges support from the Spanish Ministry Project AYA2015-71718-R (including EU funds). We thank Caisey Harlingten for the repeated use of his 50 cm telescopes in San Pedro de Atacama, Chile. We thank the Italian Telescopio Nazionale Galileo (TNG), operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. L.M. acknowledges support from the Italian Minister of Instruction, University and Research (MIUR) through FFABR 2017 fund and support from the University of Rome Tor Vergata through “Mission: Sustainability 2016” fund. The Astronomical Observatory of the Autonomous Region of the Aosta Valley (OAVdA) is managed by the Fondazione Clément Fillietroz-ONLUS, which is supported by the Regional Governmentof the Aosta Valley, the Town Municipality of Nus and the “Unité des Communes valdôtaines Mont-Émilius”. The research was partially funded by a 2016 “Research and Education”s grant from Fondazione CRT. We thank D.P. Hinson for his constructive and detailed comments that helped to improve this article., Context. The tenuous nitrogen (N2) atmosphere on Pluto undergoes strong seasonal effects due to high obliquity and orbital eccentricity, and has recently (July 2015) been observed by the New Horizons spacecraft. Aims. The main goals of this study are (i) to construct a well calibrated record of the seasonal evolution of surface pressure on Pluto and (ii) to constrain the structure of the lower atmosphere using a central flash observed in 2015. Methods. Eleven stellar occultations by Pluto observed between 2002 and 2016 are used to retrieve atmospheric profiles (density, pressure, temperature) between altitude levels of ~5 and ~380 km (i.e. pressures from ~ 10 μbar to 10 nbar). Results. (i) Pressure has suffered a monotonic increase from 1988 to 2016, that is compared to a seasonal volatile transport model, from which tight constraints on a combination of albedo and emissivity of N2 ice are derived. (ii) A central flash observed on 2015 June 29 is consistent with New Horizons REX profiles, provided that (a) large diurnal temperature variations (not expected by current models) occur over Sputnik Planitia; and/or (b) hazes with tangential optical depth of ~0.3 are present at 4–7 km altitude levels; and/or (c) the nominal REX density values are overestimated by an implausibly large factor of ~20%; and/or (d) higher terrains block part of the flash in the Charon facing hemisphere., European Research Council under the European Community’s H2020 2014-2020 ERC Grant Agreement n° 669416 “Lucky Star”, Grants 427700/2018-3, 310683/2017-3 and 473002/2013-2) and FAPERJ (Grant E-26/111.488/2013), Grant AYA2017-89637-R. P.S.S. acknowledges financial support from the European Union’s Horizon 2020 Research and Innovation Programme, under Grant Agreement no 687378, as part of the project “Small Bodies Near and Far” (SBNAF), Financial support from the State Agency for Research of the Spanish MCIU through the “Center of Excellence Severo Ochoa” award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709), Support process 309578/2017-5. G.B.R. thanks support from the grant CAPES-FAPERJ/PAPDRJ (E26/203.173/2016). J.I.B.C. acknowledges CNPq grant 308150/2016-3. R.V.M. thanks the grants: CNPq-304544/2017-5, 401903/2016-8, and Faperj: PAPDRJ-45/2013 and E-26/203.026/2015. B.M. thanks the CAPES/Cofecub-394/2016-05 grant and CAPES/Brazil – Finance Code 001., Grant FRFC 2.5.594.09.F, with the participation of the Swiss National Science Foundation (FNS/SNSF), Support from the Spanish Ministry Project AYA2015-71718-R (including EU funds), Support from the Italian Minister of Instruction, University and Research (MIUR) through FFABR 2017 fund and support from the University of Rome Tor Vergata through “Mission: Sustainability 2016” fund

The Low-Frequency Array radio telescope discovered the $707$ Hz binary millisecond pulsar (MSP) J0952$-$0607 in a targeted radio pulsation search of an unidentified $\textit{Fermi}$ gamma-ray source. This source shows a weak energy flux of $F_\gamma = 2.6 \times 10^{-12}\,\text{erg}\,\text{cm}^{-2}\,\text{s}^{-1}$ in the energy range between $100\,\text{MeV}$ and $100\,\text{GeV}$. Here we report the detection of pulsed gamma-ray emission from PSR$\,$J0952$-$0607 in a very sensitive gamma-ray pulsation search. The pulsar's rotational, binary, and astrometric properties are measured over seven years of $\textit{Fermi}$-Large Area Telescope data. For this we take into account the uncertainty on the shape of the gamma-ray pulse profile. We present an updated radio-timing solution now spanning more than two years and show results from optical modeling of the black-widow-type companion based on new multi-band photometric data taken with HiPERCAM on the Gran Telescopio Canarias on La Palma and ULTRACAM on the New Technology Telescope at ESO La Silla. PSR$\,$J0952$-$0607 is now the fastest-spinning pulsar for which the intrinsic spin-down rate has been reliably constrained ($\dot{P}_\text{int} \lesssim 4.6 \times 10^{-21}\,\text{s}\,\text{s}^{-1}$). The inferred surface magnetic field strength of $B_\text{surf} \lesssim 8.2 \times 10^{7}\,\text{G}$ is among the ten lowest of all known pulsars. This discovery is another example of an extremely fast spinning black-widow pulsar hiding within an unidentified $\textit{Fermi} gamma-ray source. In the future such systems might help to pin down the maximum spin frequency and the minimum surface magnetic field strength of MSPs., Comment: 23 pages, 8 figures

PDS 110 is a young disk-hosting star in the Orion OB1A association. Two dimming events of similar depth and duration were seen in 2008 (WASP) and 2011 (KELT), consistent with an object in a closed periodic orbit. In this paper we present data from a ground-based observing campaign designed to measure the star both photometrically and spectroscopically during the time of predicted eclipse in September 2017. Despite high-quality photometry, the predicted eclipse did not occur, although coherent structure is present suggesting variable amounts of stellar flux or dust obscuration. We also searched for RV oscillations caused by any hypothetical companion and can rule out close binaries to 0.1 $M_\odot$. A search of Sonneberg plate archive data also enabled us to extend the photometric baseline of this star back more than 50 years, and similarly does not re-detect any deep eclipses. Taken together, they suggest that the eclipses seen in WASP and KELT photometry were due to aperiodic events. It would seem that PDS 110 undergoes stochastic dimmings that are shallower and shorter-duration than those of UX Ori variables, but may have a similar mechanism., Comment: Accepted to MNRAS; 12 pages, 7 figures; Supplementary photometric data in zipped latex source as all_photometry.csv

We report on simultaneous sub-second optical and X-ray timing observations of the low mass X-ray binary black hole candidate MAXI J1820+070. The bright 2018 outburst rise allowed simultaneous photometry in five optical bands ($ugriz_s$) with HiPERCAM/GTC (Optical) at frame rates over 100 Hz, together with NICER/ISS observations (X-rays). Intense (factor of two) red flaring activity in the optical is seen over a broad range of timescales down to $\sim$10 ms. Cross-correlating the bands reveals a prominent anti-correlation on timescales of $\sim$seconds, and a narrow sub-second correlation at a lag of $\approx$+165 ms (optical lagging X-rays). This lag increases with optical wavelength, and is approximately constant over Fourier frequencies of $\sim$0.3-10 Hz. These features are consistent with an origin in the inner accretion flow and jet base within $\sim$5000 Gravitational radii. An additional $\sim$+5 s lag feature may be ascribable to disc reprocessing. MAXI J1820+070 is the third black hole transient to display a clear $\sim$0.1s optical lag, which may be common feature in such objects. The sub-second lag $variation$ with wavelength is novel, and may allow constraints on internal shock jet stratification models., MNRAS Letters, In Press

Here we confirm Melnick 34, an X-ray bright star in the 30 Doradus region of the Large Magellanic Cloud, as an SB2 binary comprising WN5h+WN5h components. We present orbital solutions using 26 epochs of VLT/UVES spectra and 22 epochs of archival Gemini/GMOS spectra. Radial-velocity monitoring and automated template fitting methods both reveal a similar high eccentricity system with a mass ratio close to unity, and an orbital period in agreement with the 155.1 +/- 1 day X-ray light curve period previously derived by Pollock et al. Our favoured solution derived an eccentricity of 0.68 +/- 0.02 and mass ratio of 0.92 +/- 0.07, giving minimum masses of Ma_sin^{3}(i) = 65 +/- 7 Msun and Mb_sin^{3}(i) = 60 +/- 7 Msun. Spectral modelling using WN5h templates with CMFGEN reveals temperatures of T ~53 kK for each component and luminosities of log(La/Lsun) = 6.43 +/- 0.08 and log(Lb/Lsun) = 6.37 +/- 0.08, from which BONNSAI evolutionary modelling gives masses of Ma = 139 (+21,-18) Msun and Mb = 127 (+17,-17) Msun and ages of ~0.6 Myrs. Spectroscopic and dynamic masses would agree if Mk34 has an inclination of i ~50{\deg}, making Mk34 the most massive binary known and an excellent candidate for investigating the properties of colliding wind binaries. Within 2-3 Myrs, both components of Mk34 are expected to evolve to stellar mass black holes which, assuming the binary system survives, would make Mk34 a potential binary black hole merger progenitor and gravitational wave source., Comment: 21 pages, 18 figures, accepted for publication in MNRAS

We have observed the eclipsing, post-common envelope white dwarf-brown dwarf binary, SDSS141126.20+200911.1, in the near-IR with the HAWK-I imager, and present here the first direct detection of the dark side of an irradiated brown dwarf in the $H$ band, and a tentative detection in the $K_s$ band. Our analysis of the lightcurves and indicates that the brown dwarf is likely to have an effective temperature of 1300 K, which is not consistent with the effective temperature of 800 K suggested by its mass and radius. As the brown dwarf is already absorbing almost all the white dwarf emission in the $K_s$ band we suggest that this inconsistency may be due to the UV-irradiation from the white dwarf inducing an artificial brightening in the $K_s$ band, similar to that seen for the similar system WD0137-349B, suggesting this brightening may be characteristic of these UV-irradiated binaries., 8 pages, 7 figures, 1 table. Accepted for publication in MNRAS

We present new time-resolved photometry of 74 cataclysmic variables (CVs), 47 of which are eclipsing. 13 of these eclipsing systems are newly discovered. For all 47 eclipsing systems we show high cadence (1-20 seconds) light curves obtained with the high-speed cameras ultracam and ultraspec. We provide new or refined ephemerides, and supply mid-eclipse times for all observed eclipses. We assess the potential for light curve modelling of all 47 eclipsing systems to determine their system parameters, finding 20 systems which appear to be suitable for future study., 33 pages (incl 17 pages appendices), 37 figures (incl 25 in appendices), accepted on 22nd November 2016 for publication in MNRAS

We have conducted a mini-survey for low-frequency radio emission from some of the closest brown dwarfs to the Sun with rapid rotation rates: SIMP J013656.5+093347, WISEPC J150649.97+702736.0, and WISEPA J174124.26+255319.5. We have placed robust 3-sigma upper limits on the flux density in the 111 - 169 MHz frequency range for these targets: WISE 1506: < 0.72 mJy; WISE 1741: < 0.87 mJy; SIMP 0136: < 0.66 mJy. At 8 hours of integration per target to achieve these limits, we find that systematic and detailed study of this class of object at LOFAR frequencies will require a substantial dedication of resources., 9 pages. MNRAS accepted

We present long-term photometric observations of the young open cluster IC 348 with a baseline time-scale of 2.4 yr. Our study was conducted with several telescopes from the Young Exoplanet Transit Initiative (YETI) network in the Bessel R band to find periodic variability of young stars. We identified 87 stars in IC 348 to be periodically variable; 33 of them were unreported before. Additionally, we detected 61 periodic non-members of which 41 are new discoveries. Our wide field of view was the key to those numerous newly found variable stars. The distribution of rotation periods in IC 348 has always been of special interest. We investigate it further with our newly detected periods but we cannot find a statistically significant bimodality. We also report the detection of a close eclipsing binary in IC 348 composed of a low-mass stellar component (M ≳ 0.09 M⊙) and a K0 pre-main-sequence star (M ≈ 2.7 M⊙). Furthermore, we discovered three detached binaries among the background stars in our field of view and confirmed the period of a fourth one.

We present a long-term programme for timing the eclipses of white dwarfs in close binaries to measure apparent and/or real variations in their orbital periods. Our programme includes 67 close binaries, both detached and semi-detached and with M-dwarfs, K-dwarfs, brown dwarfs or white dwarfs secondaries. In total, we have observed more than 650 white dwarf eclipses. We use this sample to search for orbital period variations and aim to identify the underlying cause of these variations. We find that the probability of observing orbital period variations increases significantly with the observational baseline. In particular, all binaries with baselines exceeding 10 yrs, with secondaries of spectral type K2 -- M5.5, show variations in the eclipse arrival times that in most cases amount to several minutes. In addition, among those with baselines shorter than 10 yrs, binaries with late spectral type (>M6), brown dwarf or white dwarf secondaries appear to show no orbital period variations. This is in agreement with the so-called Applegate mechanism, which proposes that magnetic cycles in the secondary stars can drive variability in the binary orbits. We also present new eclipse times of NN Ser, which are still compatible with the previously published circumbinary planetary system model, although only with the addition of a quadratic term to the ephemeris. Finally, we conclude that we are limited by the relatively short observational baseline for many of the binaries in the eclipse timing programme, and therefore cannot yet draw robust conclusions about the cause of orbital period variations in evolved, white dwarf binaries., 16 pages, 12 figures

We report results of the first five observing campaigns for the open stellar cluster NGC 7243 in the frame of project Young Exoplanet Transit Initiative (YETI). The project focuses on the monitoring of young and nearby stellar clusters, with the aim to detect young transiting exoplanets and to study other variability phenomena on time-scales from minutes to years. After five observing campaigns and additional observations during 2013 and 2014, a clear and repeating transit-like signal was detected in the light curve of J221550.6+495611. Furthermore, we detected and analysed 37 new eclipsing binary stars in the studied region. The best fit parameters and light curves of all systems are given. Finally, we detected and analysed 26 new, presumably pulsating variable stars in the studied region. The follow-up investigation of these objects, including spectroscopic measurements of the exoplanet candidate, is currently planned. (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

SDSS J1021+1744 is a detached, eclipsing white dwarf / M dwarf binary discovered in the Sloan Digital Sky Survey. Outside the primary eclipse, the light curves of such systems are usually smooth and characterised by low-level variations caused by tidal distortion and heating of the M star component. Early data on SDSS J1021+1744 obtained in June 2012 was unusual in showing a dip in flux of uncertain origin shortly after the white dwarf's eclipse. Here we present high-time resolution, multi-wavelength observations of 35 more eclipses over 1.3 years, showing that the dip has a lifetime extending over many orbits. Moreover the "dip" is in fact a series of dips that vary in depth, number and position, although they are always placed in the phase interval 1.06 to 1.26 after the white dwarf's eclipse, near the L5 point in this system. Since SDSS J1021+1744 is a detached binary, it follows that the dips are caused by the transit of the white dwarf by material around the Lagrangian L5 point. A possible interpretation is that they are the signatures of prominences, a phenomenon already known from H-alpha observations of rapidly rotating single stars as well as binaries. What makes SDSS J1021+1744 peculiar is that the material is dense enough to block continuum light. The dips appear to have finally faded out around 2015 May after the first detection by Parsons et al. in 2012, suggesting a lifetime of years., Accepted for publication in MNRAS

We present the discovery of the second binary with a Roche lobe-filling hot subdwarf transferring mass to a white dwarf (WD) companion. This 56 minute binary was discovered using data from the Zwicky Transient Facility. Spectroscopic observations reveal an He-sdOB star with an effective temperature of $T_{\rm eff}=33,700\pm1000$ K and a surface gravity of $log(g)=5.54\pm0.11$. The GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the He-sdOB star and shows an eclipse of the He-sdOB by an accretion disk as well as a weak eclipse of the WD. We infer a He-sdOB mass of $M_{\rm sdOB}=0.41\pm0.04$ M$_\odot$ and a WD mass of $M_{\rm WD}=0.68\pm0.05$ M$_\odot$. The weak eclipses imply a WD black-body temperature of $63,000\pm10,000$ K and a radius $R_{\rm WD}=0.0148\pm0.0020$ M$_\odot$ as expected for a WD of such high temperature. The He-sdOB star is likely undergoing hydrogen shell burning and will continue transferring mass for $\approx1$ Myrs at a rate of $10^{-9} M_\odot {\rm yr}^{-1}$ which is consistent with the high WD temperature. The hot subdwarf will then turn into a WD and the system will merge in $\approx30$ Myrs. We suggest that Galactic reddening could bias discoveries towards preferentially finding Roche lobe-filling systems during the short-lived shell burning phase. Studies using reddening corrected samples should reveal a large population of helium core-burning hot subdwarfs with $T_{\rm eff}\approx25,000$ K in binaries of 60-90 minutes with WDs. Though not yet in contact, these binaries would eventually come into contact through gravitational wave emission and explode as a sub-luminous thermonuclear supernova or evolve into a massive single WD., Comment: accepted for publication in ApJL, 11 pages, 5 figures and 1 tables. arXiv admin note: text overlap with arXiv:2002.01485

We report the discovery of the first short period binary in which a hot subdwarf star (sdOB) fills its Roche lobe and started mass transfer to its companion. The object was discovered as part of a dedicated high-cadence survey of the Galactic Plane named the Zwicky Transient Facility and exhibits a period of $P_{\rm orb}=39.3401(1)$ min, making it the most compact hot subdwarf binary currently known. Spectroscopic observations are consistent with an intermediate He-sdOB star with an effective temperature of $T_{\rm eff}=42,400\pm300$ K and a surface gravity of $\log(g)=5.77\pm0.05$. A high-signal-to noise GTC+HiPERCAM light curve is dominated by the ellipsoidal deformation of the sdOB star and an eclipse of the sdOB by an accretion disk. We infer a low-mass hot subdwarf donor with a mass $M_{\rm sdOB}=0.337\pm0.015$ M$_\odot$ and a white dwarf accretor with a mass $M_{\rm WD}=0.545\pm0.020$ M$_\odot$. Theoretical binary modeling indicates the hot subdwarf formed during a common envelope phase when a $2.5-2.8$ M$_\odot$ star lost its envelope when crossing the Hertzsprung Gap. To match its current $P_{\rm orb}$, $T_{\rm eff}$, $\log(g)$, and masses, we estimate a post-common envelope period of $P_{\rm orb}\approx150$ min, and find the sdOB star is currently undergoing hydrogen shell burning. We estimate that the hot subdwarf will become a white dwarf with a thick helium layer of $\approx0.1$ M$_\odot$ and will merge with its carbon/oxygen white dwarf companion after $\approx17$ Myr and presumably explode as a thermonuclear supernova or form an R CrB star., accepted for publication in ApJ, 21 pages, 12 figures and 4 tables

© 2018 SPIE. HiPERCAM is a five channel fast photometer to study high temporal variability of the universe, covering from 0.3 to 1.0 microns in five wavebands. HiPERCAM uses custom-made 2Kx1K split-frame transfer CCDs mounted in separate compact camera heads and cooled by thermoelectric coolers to 180K. The demands on the readout system are very unique to this instrument in that all five CCDs are operated in a pseudo drift window mode along with the normal windowing, binning and full-frame modes. The pseudo drift mode involves reading out small window regions from 2 quadrants of each CCD, with the possibility to exceed 1 kHz window rates per output channel. The CCDs are custom manufactured by Teledyne e2v to allow independent serial clock controls for each output. The devices are manufactured in standard and deep-depletion processes with appropriate anti-reflection coatings to achieve high quantum efficiencies in each of the five wavebands. An ESO NGC controller has been configured to control and readout all five CCDs. The data acquisition software has been modified to provide GPS timestamping of the data and access to the acquired data in real time for the data reduction software. The instrument has had its first light and first science observations on the 4.2m William Herschel Telescope, La Palma during a commissioning run in October 2017 and subsequently on the 10.4m Gran Telescopio Canarias in February 2018 and science observations in April 2018. This paper will present the details of the preamplifier electronics, configuration of the readout electronics and the data acquisition software to support the unique readout modes along with the overall performance of the instrument.

SB would like to thank Tara Murphy, Martin Bell, Paul Hancock, Keith Bannister, Chris Blake and Bing Zhang for useful discussions., We report the discovery of four Fast Radio Bursts (FRBs) in the ongoing SUrvey for Pulsars and Extragalactic Radio Bursts at the Parkes Radio Telescope: FRBs 150610, 151206, 151230 and 160102. Our real-time discoveries have enabled us to conduct extensive, rapid multimessenger follow-up at 12 major facilities sensitive to radio, optical, X-ray, gamma-ray photons and neutrinos on time-scales ranging from an hour to a few months post-burst. No counterparts to the FRBs were found and we provide upper limits on afterglow luminosities. None of the FRBs were seen to repeat. Formal fits to all FRBs show hints of scattering while their intrinsic widths are unresolved in time. FRB 151206 is at low Galactic latitude, FRB 151230 shows a sharp spectral cut-off, and FRB 160102 has the highest dispersion measure (DM = 2596.1 ± 0.3 pc cm−3) detected to date. Three of the FRBs have high dispersion measures (DM > 1500 pc cm−3), favouring a scenario where the DM is dominated by contributions from the intergalactic medium. The slope of the Parkes FRB source counts distribution with fluences >2 Jy ms is α=−2.2+0.6−1.2 and still consistent with a Euclidean distribution (α = −3/2). We also find that the all-sky rate is 1.7+1.5−0.9×103 FRBs/(4π sr)/day above ∼2Jyms and there is currently no strong evidence for a latitude-dependent FRB sky rate., The Parkes radio telescope and the Australia Telescope Compact Array are part of the Australia Telescope National Facility which is funded by the Commonwealth of Australia for operation as a National Facility managed by CSIRO. Parts of this research were conducted by the Australian Research Council Centre of Excellence for All-sky Astrophysics (CAASTRO), through project number CE110001020. The GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. VLAis run by theNational Radio Astronomy Observatory (NRAO). NRAO is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. This work was performed on the gSTAR national facility at Swinburne University of Technology. gSTAR is funded by Swinburne and the Australian Government’s Education Investment Fund. This work is also based on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. We thank the LSST Project for making their code available as free software at http://dm.lsstcorp.org. Funding from the European Research Council under the European Union’s Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 617199 (EP). Access to the Lovell Telescope is supported through an STFC consolidated grant. The 100-m telescope in Effelsberg is operation by the Max- Planck-Institut für Radioastronomie with funds of the Max-Planck Society. The Sardinia Radio Telescope (SRT) is funded by the Department of University and Research (MIUR), the Italian Space Agency (ASI), and the Autonomous Region of Sardinia (RAS) and is operated as National Facility by the National Institute for Astrophysics (INAF). TB and RWW are grateful to the STFC for financial support (grant reference ST/P000541/1). Research support to IA is provided by the Australian Astronomical Observatory. The ANTARES authors acknowledge the financial support of the funding agencies: Centre National de la Recherche Scientifique (CNRS), Commissariat à l’énergie atomique et aux énergies alternatives (CEA), Commission Européenne (FEDER fund and Marie Curie Program), Institut Universitaire de France (IUF), IdEx program and UnivEarthS Labex program at Sorbonne Paris Cité (ANR-10- LABX-0023 and ANR-11-IDEX-0005-02), Labex OCEVU (ANR- 11-LABX-0060) and the A*MIDEX project (ANR-11-IDEX-0001- 02), Région Île-de-France (DIM-ACAV), Région Alsace (contrat CPER), Région Provence-Alpes-Côte d’Azur, Département du Var and Ville de La Seyne-sur-Mer, France; Bundesministerium für Bildung und Forschung (BMBF), Germany; IstitutoNazionale di Fisica Nucleare (INFN), Italy; Stichting voor Fundamenteel Onderzoek der Materie (FOM), Nederlandse organisatie voor Wetenschappelijk Onderzoek (NWO), the Netherlands; Council of the President of the Russian Federation for young scientists and leading scientific schools supporting grants, Russia; National Authority for Scientific Research (ANCS), Romania; Ministerio de Economía y Competitividad (MINECO): Plan Estatal de Investigación (refs. FPA2015- 65150-C3-1-P, -2-P and -3-P, (MINECO/FEDER)), Severo Ochoa Centre of Excellence and MultiDark Consolider (MINECO), and Prometeo and Grisolía programs (Generalitat Valenciana), Spain; Ministry of Higher Education, Scientific Research and Professional Training, Morocco. We also acknowledge the technical support of Ifremer, AIM and Foselev Marine for the sea operation and the CC-IN2P3 for the computing facilities. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester. This research has made use of data, software and/or web tools obtained from the High Energy Astrophysics Science Archive Research Center (HEASARC), a service of the Astrophysics Science Division at NASA/GSFC and of the Smithsonian Astrophysical Observatory’s High Energy Astrophysics Division. This work is based in part on data collected at Subaru Telescope, which is operated by the National Astronomical Observatory of Japan. This paper makes use of software developed for the Large Synoptic Survey Telescope. We thank the LSST Project for making their code available as free software at http://dm.lsstcorp.org. RPE/MK gratefully acknowledges support from ERC Synergy Grant "BlackHoleCam" Grant Agreement Number 610058

We present the discovery of the shortest-period, non-interacting, white dwarf-brown dwarf post-common-envelope binary known. The K2 light curve shows the system, EPIC 21223532 has a period of 68.2 min and is not eclipsing, but does show a large reflection effect due to the irradiation of the brown dwarf by the white dwarf primary. Spectra show hydrogen, magnesium and calcium emission features from the brown dwarf's irradiated hemisphere, and the mass indicates the spectral type is likely to be L3. Despite having a period substantially lower than the cataclysmic variable period minimum, this system is likely a pre-cataclysmic binary, recently emerged from the common-envelope. These systems are rare, but provide limits on the lowest mass object that can survive common envelope evolution, and information about the evolution of white dwarf progenitors, and post-common envelope evolution., 7 pages, 6 Figures, 3 Tables, Accepted for publication in MNRAS