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T CrB is a symbiotic star that experiences nova outbursts every $\sim$ 80~yr. The next, long-anticipated nova outburst should occur during the 2024-2026 period. Here, we present results of high-resolution optical spectroscopy of T CrB in the period 2016 - 2023. In these spectra, we measured the equivalent widths of the H$\alpha$, H$\beta$, HeI and HeII emission lines. The maximum equivalent width (EW) was recorded on May 2021, when the EW of $H\alpha$ reached -44.6 \AA and H$\beta$ = -21.5 \AA. At the other extreme, the minimum of EW($H\alpha$)= -2.9 \AA was recorded in October 2023. After October 2023, the B-band emission brightened, suggesting a re-appearance of the orbital modulation. In addition to the optical data, we study the X-ray behaviour in the same period. We find a strong correlation between $EW(H\alpha)$ and X-ray flux with a correlation coefficient -0.78 and a significance of 2.6$\times 10^{-5}$., Comment: Accepted for publication in Bulgarian Astronomical Journal

We report on spin variations in the intermediate polar and cataclysmic variable CC Scl, as seen by the Transiting Exoplanet Survey Satellite (TESS). By studying both the spin period and its harmonic, we find that the spin has varied since it was first observed in 2011. We find the latest spin value for the source to be 389.473(6)s, equivalent to 0.00450779(7) days, 0.02s shorter than the first value measured. A linear fit to these and intermediate data give a rate of change of spin ~-4.26(2.66)e10^-11 and a characteristic timescale tau~2.90e10^5 years, in line with other known intermediate polars with varying spin. The spin profile of this source also matches theoretical spin profiles of high-inclination intermediate polars, and furthermore, appears to have changed in shape over a period of three years. Such `spin-up' in an intermediate polar is considered to be from mass accretion onto the white dwarf (the primary), and we note the presence of dwarf nova eruptions in this source as being a possible catalyst of the variations., Comment: 5 pages, 6 figures. Accepted into MNRAS Letters

The classical nova CP Puppis has been observed to have particularly puzzling and peculiar properties. In particular, this classical nova displays occasional bursts in its long-term ASAS-SN light curve. Here we report on 5 sectors of TESS data displaying 2 of these rapid bursts, lasting ~1 day. Based on the estimated lower energy limits of the bursts we discuss whether the bursts may be examples of micronovae resulting from localised thermonuclear explosion. Furthermore, its orbital period remains uncertain, with several inconsistent periodic signals appearing in spectroscopic and photometric observations at various wavelengths. Although we cannot unambiguously unravel the physical origin of the signals, the previously suggested nature of CP Puppis as a long orbital period system may be a viable explanation. The recurrence time of the bursts in CP Puppis, together with the unexplained variable modulations make it a prime candidate for intense monitoring., Comment: 9 pages, 5 figures, accepted for publication in MNRAS

State-of-the-art techniques to identify H\alpha emission line sources in narrow-band photometric surveys consist of searching for H\alpha excess with reference to nearby objects in the sky (position-based selection). However, while this approach usually yields very few spurious detections, it may fail to select intrinsically faint and/or rare H\alpha-excess sources. In order to obtain a more complete representation of the heterogeneous emission line populations, we recently developed a technique to find outliers relative to nearby objects in the colour-magnitude diagram (CMD-based selection). By combining position-based and CMD-based selections, we built an updated catalogue of H\alpha-excess candidates in the northern Galactic Plane. Here we present spectroscopic follow-up observations and classification of 114 objects from this catalogue, that enable us to test our novel selection method. Out of the 70 spectroscopically confirmed H\alpha emitters in our sample, 15 were identified only by the CMD-based selection, and would have been thus missed by the classic position-based technique. In addition, we explore the distribution of our spectroscopically confirmed emitters in the Gaia CMD. This information can support the classification of emission line sources in large surveys, such as the upcoming WEAVE and 4MOST, especially if augmented with the introduction of other colours., Comment: 13 pages, 8 figures

We report photometric observations of the hidden symbiotic star SU Lyn in the optical bands. In five nights we detect a weak flickering in U band with amplitude of about 0.05 magnitudes. No intranight variations are found in B, V, g' and r' bands. This is one more indication that the secondary component is a white dwarf accreting at low accretion rate. We also searched for intranight variability of a dozen related object (RR Boo, RT Boo, AM Cyg, AG Peg, BF Cyg, NQ Gem, StHa190, V627 Cas, XX Oph, FS Cet and Y Gem) in which no variability above the observational errors is detected., Comment: 8 pages, to appear in Bulgarian Astron. Journal

Nova explosions are caused by global thermonuclear runaways triggered in the surface layers of accreting white dwarfs. It has been predicted that localised thermonuclear bursts on white dwarfs can also take place, similar to Type I X-ray bursts observed in accreting neutron stars. Unexplained rapid bursts from the binary system TV Columbae, in which mass is accreted onto a moderately-strong magnetised white dwarf from a low-mass companion, have been observed on several occasions in the past $\approx40$ years. During these bursts the optical/UV luminosity increases by a factor of $>3$ in less than an hour and fades over $\approx10$ hours. Fast outflows have been observed in UV spectral lines, with velocities $>3500$ km s$^{-1}$, comparable to the escape velocity from the white dwarf surface. Here we report on optical bursts observed in TV Columbae as well as in two additional accreting systems, EI Ursae Majoris and ASASSN-19bh. The bursts have a total energy $\approx~10^{-6}$ those of classical nova explosions ("micronovae"), and bear a strong resemblance to Type I X-ray bursts. We exclude accretion or stellar magnetic reconnection events as their origin and suggest thermonuclear runaway events in magnetically-confined accretion columns as a viable explanation., Comment: 11 pages, 7 figures. Submitted on 4 October 2021. Accepted for publication in Nature on 1 February 2022

Accreting white dwarfs are often found in close binary systems with orbital periods ranging from tens of minutes to several hours. In most cases, the accretion process is relatively steady, with significant modulations only occurring on time-scales of ~days or longer. Here, we report the discovery of abrupt drops in the optical luminosity of the accreting white dwarf binary system TW Pictoris by factors up to 3.5 on time-scales as short as 30 minutes. The optical light curve of this binary system obtained by the Transiting Exoplanet Survey Satellite (TESS) clearly displays fast switches between two distinct intensity modes that likely track the changing mass accretion rate onto the white dwarf. In the low mode, the system also displays magnetically-gated accretion bursts, which implies that a weak magnetic field of the white dwarf truncates the inner disk at the co-rotation radius in this mode. The properties of the mode switching observed in TW Pictoris appear analogous to those observed in transitional millisecond pulsars, where similar transitions occur, although on timescales of ~tens of seconds. Our discovery establishes a previously unrecognised phenomenon in accreting white dwarfs and suggests a tight link to the physics governing magnetic accretion onto neutron stars., Comment: Submitted on 17 June 2021. Accepted for publication in Nature Astronomy on 18 August 2021

We present a catalogue of point-like H{\alpha}-excess sources in the Northern Galactic Plane. Our catalogue is created using a new technique that leverages astrometric and photomeric information from Gaia to select H{\alpha}-bright outliers in the INT Photometric H{\alpha} Survey of the Northern Galactic Plane (IPHAS), across the colour-absolute magnitude diagram. To mitigate the selection biases due to stellar population mixing and to extinction, the investigated objects are first partitioned with respect to their positions in the Gaia colour-absolute magnitude space, and in the Galactic coordinates space, respectively. The selection is then performed on both partition types independently. Two significance parameters are assigned to each target, one for each partition type. These represent a quantitative degree of confidence that the given source is a reliable H{\alpha}-excess candidate, with reference to the other objects in the corresponding partition. Our catalogue provides two flags for each source, both indicating the significance level of the H{\alpha}-excess. By analysing their intensity in the H{\alpha} narrow band, 28,496 objects out of 7,474,835 are identified as H{\alpha}-excess candidates with a significance higher than 3. The completeness fraction of the H{\alpha} outliers selection is between 3% and 5%. The suggested 5{\sigma} conservative cut yields a purity fraction of 81.9%., Comment: 20 pages, 23 figures

We present and analyze optical photometry and high resolution SALT spectra of the symbiotic recurrent nova V3890 Sgr at quiescence. The orbital period, P=747.6 days has been derived from both photometric and spectroscopic data. Our double-line spectroscopic orbits indicate that the mass ratio is q=M_g/M_WD=0.78+/-0.05, and that the component masses are M_WD=1.35+/-0.13 Msun, and M_g=1.05+/-0.11 Msun. The orbit inclination is approximately 67-69 degr. The red giant is filling (or nearly filling) its Roche lobe, and the distance set by its Roche lobe radius, d=9 kpc, is consistent with that resulting from the giant pulsation period. The outburst magnitude of V3890 Sgr is then very similar to those of RNe in the Large Magellanic Cloud. V3890 Sgr shows remarkable photometric and spectroscopic activity between the nova eruptions with timescales similar to those observed in the symbiotic recurrent novae T CrB and RS Oph and Z And-type symbiotic systems. The active source has a double-temperature structure which we have associated with the presence of an accretion disc. The activity would be then caused by changes in the accretion rate. We also provide evidence that V3890 Sgr contains a CO WD accreting at a high, a few 1e-8 - 1e-7 Msun/yr, rate. The WD is growing in mass, and should give rise to a Type Ia supernova within about 1,000,000 yrs - the expected lifetime of the red giant., Comment: 12 pages, 9 figures; accepted for publication in MNRAS

Besides the astrometric mission of the Gaia satellite, its repeated and high-precision measurements serve also as an all-sky photometric transient survey. The sudden brightenings of the sources are published as Gaia Photometric Science Alerts and are made publicly available allowing the community to photometrically and spectroscopically follow-up the object. The goal of this paper was to analyze the nature and derive the basic parameters of Gaia18aen, transient detected at the beginning of 2018. It coincides with the position of the emission line star WRAY 15-136. The brightening was classified as a "nova?" on the basis of subsequent spectroscopic observation. We have analyzed two spectra of Gaia18aen and collected the available photometry of the object covering the brightenings in 2018 and also the preceding and following periods of quiescence. Based on this observational data, we have derived the parameters of Gaia18aen and discussed the nature of the object. Gaia18aen is the first symbiotic star discovered by the Gaia satellite. The system is an S-type symbiotic star and consists of an M giant of a slightly super-solar metallicity, with Teff ~3500 K, a radius of ~230 R$\odot$, and a high luminosity L ~7400 L$\odot$. The hot component is a hot white dwarf. We tentatively determined the orbital period of the system ~487 days. The main outburst of Gaia18aen in 2018 was accompanied by a decrease in the temperature of the hot component. The first phase of the outburst was characterized by the high luminosity L ~27000 L$\odot$, which remained constant for about three weeks after the optical maximum, later followed by the gradual decline of luminosity and increase of temperature. Several re-brightenings have been detected on the timescales of hundreds of days., Comment: accepted for publication in Astronomy & Astrophysics; 10 pages, 10 figures, 5 tables

We performed high-resolution optical spectroscopy and X-ray observations of the recently identified Mira-type symbiotic star EF Aql. Based on high-resolution optical spectroscopy obtained with SALT, we determine the temperature ($\sim $55 000 K) and the luminosity ($\sim$ 5.3 $L_\odot$) of the hot component in the system. The heliocentric radial velocities of the emission lines in the spectra reveal possible stratification of the chemical elements. We also estimate the mass-loss rate of the Mira donor star. Our Swift observation did not detect EF Aql in X-rays. The upper limit of the X-ray observations is 10$^{-12}$ erg cm$^{-2}$ s$^{-1}$, which means that EF Aql is consistent with the faintest X-ray systems detected so far. Otherwise we detected it with the UVOT instrument with an average UVM2 magnitude of 14.05. During the exposure, EF Aql became approximately 0.2 UVM2 magnitudes fainter. The periodogram analysis of the V-band data reveals an improved period of 320.4$\pm$0.3 d caused by the pulsations of the Mira-type donor star. The spectra are available upon request from the authors., Comment: Accepted for publication in MNRAS

The substantial number of binary central stars of planetary nebulae (CSPNe) now known ($\sim$50) has revealed a strong connection between binarity and some morphological features including jets and low-ionisation structures. However, some features and asymmetries might be too complex or subtle to ascribe to binary interactions alone. A tertiary component, i.e. a triple nucleus, could be the missing ingredient required to produce these features. The only proven triple, NGC 246, is insufficient to investigate the shaping role of triple nuclei, but one straight-forward way to identify more triples is to search for binaries in nuclei with known visual companions. Here we report on the SALT HRS discovery of a 4.81 d orbital period in the CSPN of Sp 3 which has a visual companion 0.31" away. The spectroscopic distance of the visual companion agrees with distance estimates to the nebula, the GAIA DR2 parallax of the central star, and the gravity distance of the central star. This supports a physical association between the visual companion and the 4.81 d binary, making the nucleus of Sp 3 a likely triple. We determine $T_\mathrm{eff}=68^{+12}_{-6}$ kK, $\log g=4.6\pm0.2$ cm s$^{-2}$ and $v_\mathrm{rot}=80\pm20$ km s$^{-1}$ for the primary from NLTE model atmosphere analysis. The peculiar nebula presents an apparent bipolar morphology, jets and an unexpected `extreme' oxygen abundance discrepancy factor (adf) of 24.6$^{+4.1}_{-3.4}$. The adf is inconsistent with the purported trend for longer orbital period post-CE PNe to exhibit normal adfs, further highlighting selection effects in post-CE PNe. The Type-I nebular abundances of Sp 3, whose origin is often tied to more massive progenitors, are incongruous with the likely Galactic Thick Disk membership of Sp 3, possibly suggesting that rotation and binarity may play an important role in the AGB nucleosynthesis of PNe. (abridged), Comment: Accepted for publication in PASA. 23 pages, 17 figures

Hen 3-160 is reported in Belczynski et al.'s (2000) catalog as a symbiotic binary system with M7 giant donor. Using $V$- and $I$-band photometry collected over 20 years we have found that the giant is a Mira variable pulsating with 242.5-day period. The period-luminosity relation locates Hen 3-160 at the distance of about 9.4 kpc, and its Galactic coordinates ($l=267.7^{\circ}$, $b=-7.9^{\circ}$) place it $\sim$1.3 kpc above the disc. This position combined with relatively high proper motions (pm$_{\rm{RA}}=-1.5$ mas yr$^{-1}$, pm$_{\rm{DEC}}=+2.9$ mas yr$^{-1}$, Gaia DR2) indicates that Hen 3-160 has to be a Galactic extended thick-disc object. Our red optical and infrared spectra show the presence of ZrO and YO molecular bands that appear relatively strong compared to the TiO bands. Here we propose that the giant in this system is intrinsic S star, enriched in products of slow neutron capture processes occurring in its interior during an AGB phase which would make Hen 3-160 the first symbiotic system with Mira variable S star., Comment: accepted for publication at Acta Astronomica

Whether planetary nebulae (PNe) are predominantly the product of binary stellar evolution as some population synthesis models (PSM) suggest remains an open question. Around 50 short period binary central stars ($P\sim1$ d) are known, but with only four with measured orbital periods over 10 d, our knowledge is severely incomplete. Here we report on the first discovery from a systematic SALT HRS survey for long period binary central stars. We find a 142 d orbital period from radial velocities of the central star of NGC~1360, HIP~16566. NGC~1360 appears to be the product of common-envelope (CE) evolution, with nebula features similar to post-CE PNe, albeit with an orbital period considerably longer than expected to be typical of post-CE PSM. The most striking feature is a newly-identified ring of candidate low-ionisation structures (LIS). Previous spatio-kinematic modelling of the nebula gives a nebula inclination of $30\pm10$ deg, and assuming the binary nucleus is coplanar with the nebula, multi-wavelength observations best fit a more massive, evolved WD companion. A WD companion in a 142 d orbit is not the focus of many PSM, making NGC~1360 a valuable system with which to improve future PSM work. HIP~16566 is amongst many central stars in which large radial velocity variability was found by low-resolution surveys. The discovery of its binary nature may indicate long period binaries may be more common than PSM models predict., Comment: 15 pages, 10 figures, 6 tables. Accepted for publication in MNRAS

Single star evolution does not allow extremely low-mass stars to cross the classical instability strip (IS) during the Hubble time. However, within binary evolution framework low-mass stars can appear inside the IS once the mass transfer (MT) is taken into account. Triggered by a discovery of low-mass 0.26 Msun RR Lyrae-like variable in a binary system, OGLE-BLG-RRLYR-02792, we investigate the occurrence of similar binary components in the IS, which set up a new class of low-mass pulsators. They are referred to as Binary Evolution Pulsators (BEPs) to underline the interaction between components, which is crucial for substantial mass loss prior to the IS entrance. We simulate a population of 500 000 metal-rich binaries and report that 28 143 components of binary systems experience severe MT (loosing up to 90% of mass), followed by at least one IS crossing in luminosity range of RR Lyrae (RRL) or Cepheid variables. A half of these systems enter the IS before the age of 4 Gyr. BEPs display a variety of physical and orbital parameters, with the most important being the BEP mass in range 0.2-0.8 Msun, and the orbital period in range 10-2500 d. Based on the light curve only, BEPs can be misclassified as genuine classical pulsators, and as such they would contaminate genuine RRL and classical Cepheid variables at levels of 0.8% and 5%, respectively. We state that the majority of BEPs will remain undetected and we discuss relevant detection limitations., Comment: 14 pages, 6 figures, 2 tables. Accepted for publication in MNRAS

SMC 3 is one of the most interesting symbiotic stars. This binary contains a bright K-type giant transferring mass to a massive white dwarf comanion, which makes it is a very promising SN Ia candidate. We discuss the evolutionary status of the system using results of population synthesis code., Comment: Proceedings of Physics of Evolved Stars 2015: A Conference Dedicated to the Memory of Olivier Chesneau, Nice, 8-12 June 2015

Preliminary results of the ongoing search for symbiotic binary stars in the Local Group of Galaxies are presented and discussed., Comment: Proceedings of Physics of Evolved Stars 2015: A Conference Dedicated to the Memory of Olivier Chesneau, Nice, 8-12 June 2015

Most stellar remnants so far have been found in binary systems, where they interact with matter from their companions. Isolated neutron stars and black holes are difficult to find as they are dark, yet they are predicted to exist in our Galaxy in vast numbers. We explored the OGLE-III database of 150 million objects observed in years 2001-2009 and found 59 microlensing events exhibiting a parallax effect due to the Earth's motion around the Sun. Combining parallax and brightness measurements from microlensing light curves with expected proper motions in the Milky Way, we identified 13 microlensing events which are consistent with having a white dwarf, neutron star or a black hole lens and we estimated their masses and distances. The most massive of our black hole candidates has 9.3 MSun and is at a distance of 2.4 kpc. The distribution of masses of our candidates indicates a continuum in mass distribution with no mass gap between neutron stars and black holes. We also present predictions on how such events will be observed by the astrometric Gaia mission., Comment: Slightly revised version, but still 16 pages, 13 figures, 6 tables, 3 or more black holes. Accepted for publication in MNRAS on 21 Feb.2016

We report the discovery and characterisation of a deeply eclipsing AM CVn-system, Gaia14aae (= ASSASN-14cn). Gaia14aae was identified independently by the All-Sky Automated Survey for Supernovae (ASAS-SN; Shappee et al. 2014) and by the Gaia Science Alerts project, during two separate outbursts. A third outburst is seen in archival Pan-STARRS-1 (PS1; Schlafly et al. 2012; Tonry et al. 2012; Magnier et al. 2013) and ASAS-SN data. Spectroscopy reveals a hot, hydrogen-deficient spectrum with clear double-peaked emission lines, consistent with an accreting double degenerate classification. We use follow-up photometry to constrain the orbital parameters of the system. We find an orbital period of 49.71 min, which places Gaia14aae at the long period extremum of the outbursting AM CVn period distribution. Gaia14aae is dominated by the light from its accreting white dwarf. Assuming an orbital inclination of 90 degrees for the binary system, the contact phases of the white dwarf lead to lower limits of 0.78 M solar and 0.015 M solar on the masses of the accretor and donor respectively and a lower limit on the mass ratio of 0.019. Gaia14aae is only the third eclipsing AM CVn star known, and the first in which the WD is totally eclipsed. Using a helium WD model, we estimate the accretor's effective temperature to be 12900+-200 K. The three out-burst events occurred within 4 months of each other, while no other outburst activity is seen in the previous 8 years of Catalina Real-time Transient Survey (CRTS; Drake et al. 2009), Pan-STARRS-1 and ASAS-SN data. This suggests that these events might be rebrightenings of the first outburst rather than individual events., Comment: 9

We report on recent optical observations of the stellar and the nebular remnants of 22 southern post-novae. In this study, for each of our targets, we obtained and analysed long-slit spectra in the spectral range 3500-6600 A and in H$\alpha$+NII narrow-band images. The changes in the emission lines' equivalent widths with the time since the outburst agree with earlier published results of other authors. We estimated an average value $\alpha$=2.37 for the exponent of the power law fitted to the post-novae continua. Our observations clearly show the two-component structure of the V842 Cen expanding nebulae, owing to the different velocities of the ejected matter. We discovered an expanding shell around V382 Vel with an outer diameter of about 12 arcsec., Comment: 24 pages, 19 figures, A&A accepted

We present optical photometric monitoring of KT Eri (Nova Eridani 2009), a He/N very fast nova which oubursted in November 2009. Our observations include BVRcIc brightness estimations as well as monitoring of the rapid brightness variations in V band. The characteristic times of these rapid changes are studied and compared with the observed in other novae., Comment: 4 pages, 1 figure, to appear in Stella Novae: Past and Future Decades conference proceedings, P. A. Woudt & V. A. R. M. Ribeiro (eds), ASPCS