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Context. The instability strip (IS) of classical Cepheids has been extensively studied theoretically. Comparing the theoretical IS edges with those obtained empirically, using the most recent Cepheids catalogs available, can provide us with insights into the physical processes that determine the position of the IS boundaries. Aims. We aim to investigate the empirical positions of the IS of the classical Cepheids in the Large Magellanic Cloud (LMC), considering any effect that increases its width, to obtain intrinsic edges that can be compared with theoretical models. Methods. We used data of classical fundamental-mode (F) and first-overtone (1O) LMC Cepheids from the OGLE-IV variable star catalog, together with a recent high-resolution reddening map from the literature. Our final sample includes 2058 F and 1387 1O Cepheids. We studied their position on the Hertzsprung-Russell diagram and determined the IS borders by tracing the edges of the color distribution along the strip. Results. We obtained the blue and red edges of the IS in V- and I-photometric bands, in addition to log Teff and log L. The results obtained show a break located at the Cepheids' period of about three days, which was not reported before. We compared our empirical borders with theoretical ones published in the literature, obtaining a good agreement for specific parameter sets. Conclusions. The break in the IS borders is most likely explained by the depopulation of second- and third-crossing classical Cepheids in the faint part of the IS, since blue loops of evolutionary tracks in this mass range do not extend blueward enough to cross the IS at the LMC metallicity. Results from the comparison of our empirical borders with theoretical ones prove that our empirical IS is a useful tool for constraining theoretical models. [ABSTRACT FROM AUTHOR]

Photometric reverberation mapping can detect the radial extent of the accretion disc (AD) in Active Galactic Nuclei by measuring the time delays between light curves observed in different continuum bands. Quantifying the constraints on the efficiency and accuracy of the delay measurements is important for recovering the AD size-luminosity relation, and potentially using quasars as standard candles. We have explored the possibility of determining the AD size of quasars using next-generation Big Data surveys. We focus on the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory, which will observe several thousand quasars with the Deep Drilling Fields and up to 10 million quasars for the main survey in six broadband filter during its 10-yr operational lifetime. We have developed extensive simulations that take into account the characteristics of the LSST survey and the intrinsic properties of the quasars. The simulations are used to characterize the light curves from which AD sizes are determined using various algorithms. We find that the time delays can be recovered with an accuracy of 5 and 15 per cent for light curves with a time sampling of 2 and 5 d, respectively. The results depend strongly on the redshift of the source and the relative contribution of the emission lines to the bandpasses. Assuming an optically thick and geometrically thin AD, the recovered time-delay spectrum is consistent with black hole masses derived with 30 per cent uncertainty. [ABSTRACT FROM AUTHOR]

Context. The Baade-Wesselink (BW) method of distance determination of Cepheids is used to calibrate the distance scale. Various versions of this method are mainly based on interferometry and/or the surface-brightness color relation (SBCR). Aims. We quantify the impact of the SBCR, its slope, and its zero point on the projection factor. This quantity is used to convert the pulsation velocity into the radial velocity in the BW method. We also study the impact of extinction and of a potential circumstellar environment on the projection factor. Methods. We analyzed HARPS-N spectra of η Aql to derive its radial velocity curve using different methods. We then applied the inverse BW method using various SBCRs in the literature in order to derive the BW projection factor. Results. We find that the choice of the SBCR is critical: a scatter of about 8% is found in the projection factor for different SBCRs in the literature. The uncertainty on the coefficients of the SBCR affects the statistical precision of the projection factor only little (1–2%). Confirming previous studies, we find that the method with which the radial velocity curve is derived is also critical, with a potential difference on the projection factor of 9%. An increase of 0.1 in E(B − V) translates into a decrease in the projection factor of 3%. A 0.1 mag effect of a circumstellar envelope (CSE) in the visible domain is rather small on the projection factor, about 1.5%. However, we find that a 0.1 mag infrared excess in the K band due to a CSE can increase the projection factor by about 6%. Conclusions. The impact of the surface-brightness color relation on the BW projection factor is found to be critical. Efforts should be devoted in the future to improve the SBCR of Cepheids empirically, but also theoretically, taking their CSE into account as well. [ABSTRACT FROM AUTHOR]

Author(s): J.-P. Beaulieu (corresponding author) [1, 4]; D. P. Bennett [1, 3, 5]; P. Fouqué [1, 6]; A. Williams [1, 7]; M. Dominik [1, 8]; U. G. Jørgensen [1, 9]; [...]

We report results from a search for Galactic high-mass eclipsing binaries. The photometric monitoring campaign was performed in Sloan r and i with the robotic twin refractor RoBoTT at the Universitätssternwarte Bochum in Chile and complemented by Johnson UBV data. Comparison with the SIMBAD data base reveals 260 variable high-mass stars. Based on well-sampled light curves, we discovered 35 new eclipsing high-mass systems and confirm the properties of six previously known systems. For all objects, we provide the first light curves and determine orbital periods through the Lafler–Kinman algorithm. Apart from GSC 08173-0018 and Pismis 24-13 (⁠|$P = 19.47\, d$| and |$20.14\, d$|⁠) and the exceptional short-period system TYC 6561-1765-1 (⁠|$P = 0.71\, d$|⁠), all systems have orbital periods between 1 and 9 d. We model the light curves of 26 systems within the framework of the Roche geometry and calculate fundamental parameters for each system component. The Roche lobe analysis indicates that 14 systems have a detached geometry, while 12 systems have a semidetached geometry; seven of them are near-contact systems. The deduced mass ratios q  = M 2/ M 1 reach from 0.4 to 1.0 with an average value of 0.8. The similarity of masses suggests that these high-mass binaries were created during the star formation process rather than by tidal capture. [ABSTRACT FROM AUTHOR]

We present the results of a 2 yr optical continuum photometric reverberation mapping campaign carried out on the nucleus of the Seyfert-1 galaxy Mrk509. Specially designed narrow-band filters were used in order to mitigate the line and pseudo-continuum contamination of the signal from the broad-line region, while allowing for high-accuracy flux calibration over a large field of view. We obtained light curves with a sub-day time sampling and typical flux uncertainties of |$1{{\ \rm per\ cent}}$|⁠. The high photometric precision allowed us to measure interband continuum time delays of up to ∼2 d across the optical range. The time delays are consistent with the relation τ ∝ λ4/3 predicted for an optically thick and geometrically thin accretion disc model. The size of the disc is, however, a factor of 1.8 larger than predictions based on the standard thin-disc theory. We argue that, for the particular case of Mrk509, a larger black hole mass due to the unknown geometry scaling factor can reconcile the difference between the observations and theory. [ABSTRACT FROM AUTHOR]

We present candidates for non-pulsating stars lying in the classical Cepheid instability strip based on the Optical Gravitational Lensing Experiment (OGLE) photometric maps combined with Strömgren photometry obtained with the 4.1-m Southern Astrophysical Research (SOAR) telescope, and Gaia Data Release 2 data in four fields in the Large Magellanic Cloud. We selected 19 candidates in total. After analysis of their light curves from OGLE surveys we found that all these stars appear to be photometrically stable at the level of a few mmag. Our results show that non-pulsating stars might constitute to about |$21\!-\!30\, {\rm per\, cent}$| of the whole sample of giant stars located in the classical instability strip. Furthermore, we identified potential candidates for classical Cepheids with hot companions based on their Strömgren colours. [ABSTRACT FROM AUTHOR]

Context. Accurate radial velocities (vrad) of Cepheids are mandatory within the context of Cepheid distance measurements using the Baade-Wesselink technique. The most common vrad derivation method consists in cross-correlating the observed stellar spectra with a binary template and measuring a velocity on the resulting mean profile. Nevertheless, for Cepheids and other pulsating stars, the spectral lines selected within the template as well as the way of fitting the cross-correlation function (CCF) have a direct and significant impact on the measured vrad. Aims. Our first aim is to detail the steps to compute consistent CCFs and vrad of Cepheids. Next, this study aims at characterising the impact of Cepheid spectral properties and vrad computation methods on the resulting line profiles and vrad time series. Methods. We collected more than 3900 high-resolution spectra from seven different spectrographs of 64 Classical Milky Way (MW) Cepheids. These spectra were normalised and standardised using a single custom-made process on pre-defined wavelength ranges. We built six tailored correlation templates selecting unblended spectral lines of different depths based on a synthetic Cepheid spectrum, on three different wavelength ranges from 3900 to 8000 Å. Each observed spectrum was cross-correlated with these templates to build the corresponding CCFs, adopted as the proxy for the spectrum mean line profile. We derived a set of line profile observables as well as three different vrad measurements from each CCF and two custom proxies for the CCF quality and amount of signal. Results. This study presents a large catalogue of consistent Cepheid CCFs and vrad time series. It confirms that each step of the process has a significant impact on the deduced vrad: the wavelength, the template line depth and width, and the vrad computation method. The way towards more robust Cepheid vrad time series seems to go through steps that minimise the asymmetry of the line profile and its impact on the vrad. Centroid or first-moment vrad, that exhibit slightly smaller amplitudes but significantly smaller scatter than Gaussian or biGaussian vrad, should therefore be favoured. Stronger or deeper spectral lines also tend to be less asymmetric and lead to more robust vrad than weaker lines. [ABSTRACT FROM AUTHOR]

Aims. Our aim is to obtain high-accuracy measurements of the physical and orbital parameters of two evolved eclipsing binary systems, and to use these measurements to study their evolutionary status. We also aim to derive the distances to the systems by using a surface brightness–colour relation and compare these distances with the measurements provided by Gaia. Methods. We measured the physical and orbital parameters of both systems based on V-band and I-band photometry from OGLE, near-infrared photometry obtained with the NTT telescope and the instrument SOFI, as well as high-resolution spectra obtained at ESO 3.6m/HARPS and Clay 6.5/MIKE spectrographs. The light curves and radial-velocity curves were analysed with the Wilson–Devinney code. Results. We analysed two double-lined eclipsing binary systems OGLE-BLG-ECL-123903 and OGLE-BLG-ECL-296596 from the Optical Gravitational Lensing Experiment (OGLE) catalogue. Both systems have a configuration of two well-detached giants stars. The masses of the components of OGLE-BLG-ECL-123903 are M1 = 2.045 ± 0.027 and M2 = 2.074 ± 0.023 M⊙ and the radii are R1 = 9.540 ± 0.049 and R2 = 9.052 ± 0.060 R⊙. For OGLE-BLG-ECL-296596, the masses are M1 = 1.093 ± 0.015 and M2 = 1.125 ± 0.014 M⊙, while the radii are R1 = 18.06 ± 0.28 and R2 = 29.80 ± 0.33 R⊙. Evolutionary status was discussed based on the isochrones and evolutionary tracks from PARSEC and MESA codes. The ages of the systems were established to be around 1.3 Gyr for the OGLE-BLG-ECL-123903 and 7.7 Gyr for the OGLE-BLG-ECL-296596. We also determined the distance to both systems. For OGLE-BLG-ECL-123903 this is equal to d = 2.95 ± 0.06 (stat.) ±0.07 (syst.) kpc, while for the OGLE-BLG-ECL-296596 it is d = 5.68 ± 0.07 (stat.) ±0.14 (syst.) kpc. This is the first analysis of its kind for these unique evolved eclipsing binary systems. [ABSTRACT FROM AUTHOR]

Context. The Baade-Wesselink (BW) method, which combines linear and angular diameter variations, is the most common method to determine the distances to pulsating stars. However, the projection factor, p-factor, used to convert radial velocities into pulsation velocities, is still poorly calibrated. This parameter is critical on the use of this technique, and often leads to 5-10% uncertainties on the derived distances. Aims. We focus on empirically measuring the p-factor of a homogeneous sample of 29 LMC and 10 SMC Cepheids for which an accurate average distances were estimated from eclipsing binary systems. Methods. We used the SPIPS algorithm, which is an implementation of the BW technique. Unlike other conventional methods, SPIPS combines all observables, i.e. radial velocities, multi-band photometry and interferometry into a consistent physical modelling to estimate the parameters of the stars. The large number and their redundancy insure its robustness and improves the statistical precision. Results. We successfully estimated the p-factor of several Magellanic Cloud Cepheids. Combined with our previous Galactic results, we find the following P-p relation: -0:08±0:04(log P - 1:18) + 1:24±0:02. We find no evidence of a metallicity dependent p-factor. We also derive a new calibration of the period-radius relation, log R = 0:684±0:007(log P-0:517)+1:489±0:002, with an intrinsic dispersion of 0.020. We detect an infrared excess for all stars at 3:6 μm and 4:5 μm, which might be the signature of circumstellar dust. We measure a mean offset of Δm3.6 = 0:057 ± 0:006 mag and Δm4.5 = 0:065 ± 0:008 mag. Conclusions. We provide a new P-p relation based on a multi-wavelength fit that can be used for the distance scale calibration from the BW method. The dispersion is due to the LMC and SMC width we took into account because individual Cepheids distances are unknown. The new P-R relation has a small intrinsic dispersion: 4.5% in radius. This precision will allow us to accurately apply the BW method to nearby galaxies. Finally, the infrared excesses we detect again raise the issue of using mid-IR wavelengths to derive period-luminosity relation and to calibrate the Hubble constant. These IR excesses might be the signature of circumstellar dust, and are never taken into account when applying the BW method at those wavelengths. Our measured offsets may give an average bias of ~2:8% on the distances derived through mid-IR P-L relations. [ABSTRACT FROM AUTHOR]

We present K-band Multi-Object Spectrograph (KMOS) observations of 18 red supergiant (RSG) stars in the Sculptor Group galaxy NGC 55. Radial velocities are calculated and are shown to be in good agreement with previous estimates, confirming the supergiant nature of the targets and providing the first spectroscopically confirmedRSGs inNGC55. Stellar parameters are estimated for 14 targets using the J-band analysis technique, making use of state-of-theart stellar model atmospheres. The metallicities estimated confirm the low-metallicity nature of NGC 55, in good agreement with previous studies. This study provides an independent estimate of the metallicity gradient of NGC 55, in excellent agreement with recent results published using hot massive stars. In addition, we calculate luminosities of our targets and compare their distribution of effective temperatures and luminosities to that of other RSGs, in different environments, estimated using the same technique. [ABSTRACT FROM AUTHOR]

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 M☉) RR Lyrae-like variable in a binary system, OGLE-BLGRRLYR-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 (losing up to 90 per cent 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 M☉, and the orbital period in range 10-2 500 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 per cent, respectively. We state that the majority of BEPs will remain undetected and we discuss relevant detection limitations. [ABSTRACT FROM AUTHOR]

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 M☉) RR Lyrae-like variable in a binary system, OGLE-BLGRRLYR- 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 (losing up to 90 per cent 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 M☉, and the orbital period in range 10-2 500 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 per cent, respectively. We state that the majority of BEPs will remain undetected and we discuss relevant detection limitations. [ABSTRACT FROM AUTHOR]

We report new CHARA/Michigan InfraRed Combiner interferometric observations of the Cepheid archetype δ Cep, which aimed at detecting the newly discovered spectroscopic companion. We reached a maximum dynamic range ΔH = 6.4, 5.8 and 5.2 mag, respectively, within the relative distance to the Cepheid r < 25 mas, 25 < r < 50 mas and 50 < r < 100 mas. Our observations did not show strong evidence of a companion. We have a marginal detection at 3s with a flux ratio of 0.21 per cent, but nothing convincing as we found other possible probable locations. We ruled out the presence of companion with a spectral type earlier than F0V, A1V and B9V, respectively for the previously cited ranges r. From our estimated sensitivity limits and the Cepheid light curve, we derived lower limit magnitudes in the H band for this possible companion to be Hcomp > 9.15, 8.31 and 7.77 mag, respectively, for r < 25 mas, 25 < r < 50 mas and 50 < r < 100 mas. We also found that to be consistent with the predicted orbital period (Anderson et al.), the companion has to be located at a projected separation <24 mas with a spectral type later than an F0V star. [ABSTRACT FROM AUTHOR]

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 data base 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 M⊙ 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. [ABSTRACT FROM AUTHOR]

We present a statistical analysis of the first four seasons from a 'second-generation' microlensing survey for extrasolar planets, consisting of near-continuous time coverage of 8 deg² of the Galactic bulge by the Optical Gravitational Lens Experiment (OGLE), Microlensing Observations in Astrophysics (MOA), and Wise microlensing surveys. During this period, 224 microlensing events were observed by all three groups. Over 12 per cent of the events showed a deviation from single-lens microlensing, and for ~one-third of those the anomaly is likely caused by a planetary companion. For each of the 224 events, we have performed numerical ray-tracing simulations to calculate the detection efficiency of possible companions as a function of companion-to-host mass ratio and separation. Accounting for the detection efficiency, we find that 55-22+34 per cent of microlensed stars host a snowline planet. Moreover, we find that Neptune-mass planets are ~10 times more common than Jupiter-mass planets. The companion-to-host mass-ratio distribution shows a deficit at q ~ 10-2, separating the distribution into two companion populations, analogous to the stellar-companion and planet populations, seen in radial-velocity surveys around solar-like stars. Our survey, however, which probes mainly lower mass stars, suggests a minimum in the distribution in the super-Jupiter mass range, and a relatively high occurrence of brown-dwarf companions. [ABSTRACT FROM AUTHOR]

We use six years (2003-2008) of Optical Gravitational Lensing Experiment-III microlensing observations to derive the survey detection efficiency for a range of planetary masses and projected distances from the host star. We perform an independent analysis of the microlensing light curves to extract the event parameters and compute the planet detection probability given the data. 2433 light curves satisfy our quality selection criteria and are retained for further processing. The aggregate of the detection probabilities over the range explored yields the expected number of microlensing planet detections. We employ a Galactic model to convert this distribution from dimensionless to physical units, α/au and M⊕. The survey sensitivity to small planets is highest in the range 1-4 au, shifting to slightly larger separations for more massive ones. [ABSTRACT FROM AUTHOR]

Aims. Our aim is to precisely measure the physical parameters of the eclipsing binary IO Aqr and derive a distance to this system by applying a surface brightness -- colour relation. Our motivation is to combine these parameters with future precise distance determinations from the Gaia space mission to derive precise surface brightness -- colour relations for stars. Methods. We extensively used photometry from the Super-WASP and ASAS projects and precise radial velocities obtained from HARPS and CORALIE high-resolution spectra. We analysed light curves with the code JKTEBOP and radial velocity curves with the Wilson-Devinney program. Results. We found that IO Aqr is a hierarchical triple system consisting of a double-lined short-period (P = 2.37 d) spectroscopic binary and a low-luminosity and low-mass companion star orbiting the binary with a period of ≳25 000 d (≳70 yr) on a very eccentric orbit. We derive high-precision (better than 1%) physical parameters of the inner binary, which is composed of two slightly evolved main-sequence stars (F5 V-IV + F6 V-IV) with masses of M1 = 1.569 ± 0.004 and M2 = 1.655 ± 0.004 M⊙ and radii R1 = 2.19 ± 0.02 and R2 = 2.49 ± 0.02 R⊙. The companion is most probably a late K-type dwarf with mass ≈0.6 M⊙. The distance to the system resulting from applying a () surface brightness -- colour relation is 255 ± 6 (stat.) ± 6 (sys.) pc, which agrees well with the HIPPARCOS value of 270+91-55 pc, but is more precise by a factor of eight. [ABSTRACT FROM AUTHOR]

We have analyzed the double-lined eclipsing binary system ASAS J180057-2333.8 from the All Sky Automated Survey (ASAS) catalogue. We measure absolute physical and orbital parameters for this system based on archival V-band and I-band ASAS photometry, as well as on high-resolution spectroscopic data obtained with ESO 3.6 m/HARPS and CORALIE spectrographs. The physical and orbital parameters of the system were derived with an accuracy of about 0.5-3 per cent. The system is a very rare configuration of two bright welldetached giants of spectral types K1 and K4 and luminosity class II. The radii of the stars are R1 = 52.12 ± 1.38 and R2 = 67.63 ± 1.40 R☉ and their masses are M1 = 4.914 ± 0.021 and M2 = 4.875 ± 0.021 M☉. The exquisite accuracy of 0.5 per cent obtained for the masses of the components is one of the best mass determinations for giants. We derived a precise distance to the system of 2.14 ± 0.06 kpc (stat.) ± 0.05 (syst.) which places the star in the Sagittarius-Carina arm. The Galactic rotational velocity of the star is Θs = 258 ± 26 km s-1 assuming Θ0 = 238 km s-1. A comparison with PARSEC isochrones places the system at the early phase of core helium burning with an age of slightly larger than 100 million years. The effect of overshooting on stellar evolutionary tracks was explored using the MESA star code. [ABSTRACT FROM AUTHOR]

Context. The eclipsing binary method for determining distance in the local group is based on the surface brightness-color relation (SBCR), and early-type stars are preferred targets because of their intrinsic brightness. However, this type of star exhibits wind, massloss, pulsation, and rotation, which may generate bias on the angular diameter determination. An accurate calibration of the SBCR relation thus requires careful analysis. Aims. In this paper we aim to quantify the impact of stellar rotation on the SBCR when the calibration of the relation is based on interferometric measurements of angular diameters. Methods. Six stars with V - K color indices ranging between -1 and 0.5 were modeled using the code for high angular resolution of rotating objects in nature (CHARRON) with various rotational velocities (0, 25, 50, 75, and 95% of the critical rotational velocity) and inclination (0, 25, 50, 75, and 90 degrees). All these models have their equatorial axis aligned in an east-west orientation in the sky. We then simulated interferometric observations of these theoretical stars using three representative sets of the CHARA baseline configurations. The simulated data were then interpreted as if the stars were non-rotating to determine an angular diameter and estimate the surface-brightness relation. The V - K color of the rotating star was calculated directly from the CHARRON code. This provides an estimate of the intrinsic dispersion of the SBCR relation when the rotation e ects of flattening and gravity darkening are not considered in the analysis of interferometric data. Results. We find a clear relation between the rotational velocity and (1) the shift in zero point (Δa0) of the SBCR (compared to the static relation) and (2) its dispersion (σ). When considering stars rotating at less than 50% of their critical velocity, Δa0 and σ have about 0.01 mag, while these quantities can reach 0.08 and 0.04 mag, respectively, when the rotation is larger than 75% of the critical velocity. Besides this, the inclination angle mostly has an impact on the V - K color: i < 50° (resp. i > 50°) makes the star redder (resp. bluer). When considering the 150 models, Δa0 and σ have 0.03 and 0.04 mag, respectively. These values are slightly but not significantly modified (about 0.03 and 0.01 mag in Δa0 and σ, respectively) when considering di erent CHARA configurations. Interestingly, these 150 models, regardless of the interferometric configuration, are consistent with the empirical SBCR, which is within its dispersion of 0:16 mag. In addition, if one only considers projected rotational velocity Vrot sin i lower than 100 km s-1, then Δa0 and σ have 0.02 and 0.03 mag, respectively. Conclusions. To calibrate the SBCR interferometrically at the 0.02 mag precision (or lower), one should consider (1) a baseline configuration covering all directions of the (u, v) plan; (2) a sample of stars with rotational velocity lower than 50% of their critical velocity or, alternatively, stars with Vrot sin i lower than 100 km s-1; (3) homogeneous visible and infrared photometry precisely at the 0.02 mag level or lower. [ABSTRACT FROM AUTHOR]

Context. Long-baseline interferometry is an important technique to spatially resolve binary or multiple systems in close orbits. By combining several telescopes together and spectrally dispersing the light, it is possible to detect faint components around bright stars in a few hours of observations. Aims. We provide a rigorous and detailed method to search for high-contrast companions around stars, determine the detection level, and estimate the dynamic range from interferometric observations. Methods. We developed the code CANDID (Companion Analysis and Non-Detection in Interferometric Data), a set of Python tools that allows us to search systematically for point-source, high-contrast companions and estimate the detection limit using all interferometric observables, i.e., the squared visibilities, closure phases and bispectrum amplitudes. The search procedure is made on a N × N grid of fit, whose minimum needed resolution is estimated a posteriori. It includes a tool to estimate the detection level of the companion in the number of sigmas. The code CANDID also incorporates a robust method to set a 3σ detection limit on the flux ratio, which is based on an analytical injection of a fake companion at each point in the grid. Our injection method also allows us to analytically remove a detected component to 1) search for a second companion; and 2) set an unbiased detection limit. Results.We used CANDID to search for the companions around the binary Cepheids V1334 Cyg, AX Cir, RT Aur, AWPer, SU Cas, and T Vul. First, we showed that our previous discoveries of the components orbiting V1334 Cyg and AX Cir were detected at >25σ and >13σ, respectively. The astrometric positions and flux ratios provided by CANDID for these two stars are in good agreement with our previously published values. The companion around AW Per is detected at more than 15σ with a flux ratio of f = 1:22 ± 0:30%, and it is located at ρ = 32:16 ± 0:29 mas and PA = 67:1 ± 0:3°. We made a possible detection of the companion orbiting RT Aur with f = 0:22 ± 0:11%, and at ρ = 2:10 ± 0:23 mas and PA = -136 ± 6°. It was detected at 3.8σ using the closure phases only, and so more observations are needed to confirm the dectection. No companions were detected around SU Cas and T Vul. We also set the detection limit for possible undetected companions around these stars. We found that there is no companion with a spectral type earlier than B7V, A5V, F0V, B9V, A0V, and B9V orbiting the Cepheids V1334 Cyg, AX Cir, RT Aur, AW Per, SU Cas, and T Vul, respectively. This work also demonstrates the capabilities of the MIRC and PIONIER instruments, which can reach a dynamic range of 1:200, depending on the angular distance of the companion and the (u, v) plane coverage. In the future, we plan to work on improving the sensitivity limits for realistic data through better handling of the correlations. [ABSTRACT FROM AUTHOR]

We present the first full orbital and physical analysis of HD 187669, recognized by the All-Sky Automated Survey (ASAS) as the eclipsing binary ASAS J195222-3233.7. We combined multi-band photometry from the ASAS and Super WASP public archives and 0.41-m PROMPT robotic telescopes with our high-precision radial velocities from the HARPS spectrograph. Two different approaches were used for the analysis: (1) fitting to all data simultaneously with the WD code and (2) analysing each light curve (with JKTEBOP) and radial velocities separately and combining the partial results at the end. This system also shows a total primary (deeper) eclipse, lasting for about 6 d. A spectrum obtained during this eclipse was used to perform atmospheric analysis with the MOOG and SME codes to constrain the physical parameters of the secondary. We found that ASAS J195222-3233.7 is a double-lined spectroscopic binary composed of two evolved, late-type giants, with masses of M1 = 1.504 ± 0.004 and M2 = 1.505 ± 0.004 M8857;, and radii of R1 = 11.33 ± 0.28 and R2 = 22.62 ± 0.50 R8857;. It is slightly less metal abundant than the Sun, and has a P = 88.39 d orbit. Its properties are well reproduced by a 2.38-Gyr isochrone, and thanks to the metallicity estimation from the totality spectrum and high precision of the masses, it was possible to constrain the age down to 0.1 Gyr. It is the first so evolved Galactic eclipsing binary measured with such good accuracy, and as such it is a unique benchmark for studying the late stages of stellar evolution. [ABSTRACT FROM AUTHOR]

We present the analysis of Blazhko-type modulation in double-mode RR Lyrae (RRd) stars found in the Optical Gravitational Lensing Experiment (OGLE) photometry of the Galactic bulge. Modulation is detected in 15 stars. Most of them have non-typical period ratio of the radial modes. In the Petersen diagram, at a given period of the fundamental mode, they are located significantly below or above the sequence formed by the majority of RRd stars. Multiperiodic modulation is very frequent; two or three modulation periods are detected in eight stars. Modulation periods vary from ∼20 to more than 300 d. Radial mode amplitudes can be modulated by a few to nearly 100 per cent. Both radial modes may be modulated with the same period. More commonly however, dominant modulation for the fundamental mode has different period than dominant modulation for the first overtone. Quite often modulation of only one mode is detected in the data. We find a clear feedback between pulsation amplitude of the dominant mode and mean stellar brightness: the lower the pulsation amplitude, the brighter the star. At phases of low pulsation amplitude, the mode periods are prone to fast changes. All the stars share the common feature: their pulsation properties are non-stationary. Amplitudes and phases of the radial modes vary irregularly on a long time-scale of a few hundred or thousand days. The short-term modulations are also irregular. One of the stars has switched the pulsation mode recently: from single-mode fundamental mode pulsation to RRd state. In other star the non-radial mode with characteristic ∼0.61 period ratio to the first overtone is detected. This non-radial mode is likely modulated with the same period as the radial modes. [ABSTRACT FROM AUTHOR]

We report the discovery of an intriguing triple-mode RR Lyrae star found in the Optical Gravitational Lensing Experiment (OGLE) Galactic bulge collection, OGLE-BLG-RRLYR-24137. In the OGLE catalogue, the star was identified as RRd star – double-mode pulsator, pulsating simultaneously in the fundamental and in the first overtone modes. We find that third mode is excited and firmly detect its period doubling. Period ratios are not far from that expected for triple-mode – fundamental, first and third overtone – pulsation. Unfortunately, we cannot reproduce period ratios of the three modes with a consistent set of pulsation models. Therefore the other interpretation, that additional mode is non-radial, is also likely. [ABSTRACT FROM AUTHOR]

Context. The method of distance determination of eclipsing binaries consists in combining the radii of both components determined from spectro-photometric observations with their respective angular diameters derived from the surface brightness-color relation (SBC). However, the largest limitation of the method comes from the uncertainty on the SBC relation: about 2% for late-type stars (or 0.04 magnitude) and more than 10% for early-type stars (or 0.2 mag). Aims. The aim of this work is to improve the SBC relation for early-type stars in the -1 ⩽ V - K ⩽ 0 color domain, using optical interferometry. Methods. Observations of eight B- and A-type stars were secured with the VEGA/CHARA instrument in the visible. The derived uniform disk angular diameters were converted into limb darkened angular diameters and included in a larger sample of 24 stars, already observed by interferometry, in order to derive a revised empirical relation for O, B, A spectral type stars with a V - K color index ranging from -1 to 0. We also took the opportunity to check the consistency of the SBC relation up to V - K ≃ 4 using 100 additional measurements. Results. We determined the uniform disk angular diameter for the eight following stars: γ Ori, ζ Per, 8 Cyg, ι Her, λ Aql, ζ Peg, γ Lyr, and δ Cyg with V - K color ranging from -0.70 to 0.02 and typical precision of about 1.5%. Using our total sample of 132 stars with V - K colors index ranging from about -1 to 4, we provide a revised SBC relation. For late-type stars (0 ⩽ V - K ⩽ 4), the results are consistent with previous studies. For early-type stars (-1 ⩽ V - K ⩽ 0), our new VEGA/CHARA measurements combined with a careful selection of the stars (rejecting stars with environment or stars with a strong variability), allows us to reach an unprecedented precision of about 0.16 magnitude or ≃7% in terms of angular diameter. Conclusions. We derive for the first time a SBC relation for stars between O9 and A3, which provides a new and reliable tool for the distance scale calibration. [ABSTRACT FROM AUTHOR]

We analyse long-term optical photometry for two Galactic recurrent novae (V745 Sco and V3890 Sgr) and one Large Magellanic Cloud object (Nova LMC 1990b) observed over several years by the Optical Gravitational Lensing Experiment (OGLE) sky survey. We do not find variability with the previously claimed orbital period of V745 Sco. This voids previous findings based on this value (e.g. the distance determination). The quiescence variability of this object is dominated by semiregular pulsations of the red giant secondary (with periods of 136.5 and 77.4 d). The photometry of Nova LMC 1990b reveals an unnoticed eruption in 2010 and eclipse-like variability in quiescence with a period of 1.26432(8) d. The photometric properties make this object very similar to U Sco. Finally, we describe the eruptions of two likely symbiotic novae, V5590 Sgr and OGLE-2011-BLG-1444. The secondary of V5590 Sgr is a Mira star with pulsation period of 236 d. [ABSTRACT FROM PUBLISHER]

Aims. High-resolution imaging in several photometric bands can provide color and astrometric information of the wide-orbit component of Cepheid stars. Such measurements are needed to understand the age and evolution of pulsating stars. In addition, binary Cepheids have the potential to provide direct and model-independent distances and masses. Methods. We used the NAOS-CONICA adaptive optics instrument (NACO) in the near-infrared to perform a deep search for wide components around the classical Cepheids, Y Oph, FF Aql, X Sgr,WSgr, and η Aql, within a field of view of 1.7" × 1.7" (3.4" × 3.4" for Aql). Results. We were able to reach contrast ΔH = 5-8 mag and ΔKs = 4-7 mag in the radius range r > 0.2", which enabled us to constrain the presence of wide companions. For Y Oph, FF Aql, X Sgr, W Sgr, and Aql at r > 0.2", we ruled out the presence of companions with a spectral type that is earlier than a B7V, A9V, A9V, A1V, and G5V star, respectively. For 0.1" < r < 0.2", no companions earlier than O9V, B3V, B4V, B2V, and B2V star, respectively, are detected. A component is detected close to Aql at projected separation ρ= 654.7 ± 0.9 mas and a position angle PA = 92.8 ± 0.1. We estimated its dereddened apparent magnitude to be mH0 = 9.34 ± 0.04 and derived a spectral type that ranges between an F1V and an F6V star. Additional photometric and astrometric measurements are necessary to better constrain this star and check its physical association to the η Aql system. [ABSTRACT FROM AUTHOR]

We present a variability study of 4646 massive stars in the Small Magellanic Cloud (SMC) with known spectral types from the catalog of Bonanos et al. (2010) using the light curves from the OGLE-III database. The goal is to exploit the time domain information available through OGLE-III to gain insight into the processes that govern the evolution of massive stars. This variability survey of massive stars with known spectral types is larger than any previous survey by a factor of 7. We find that 60% of our sample (2766 stars) show no significant variability and 40% (1880 stars) exhibit variability distributed as follows: 807 stars display low-amplitude stochastic variability with fluctuations in the I-band of up to 0.05 mag, 443 stars present irregular variability of higher amplitude (76% of these are reported as variables for the first time), 205 are eclipsing binaries (including 101 newly discovered systems), 50 are candidate rotating variables, 126 are classical Cepheids, 188 stars exhibit short-term sinusoidal periodicity (P < 3 days) making them candidate "slowly pulsating B stars" and non-radial Be pulsators, and 61 periodic stars exhibit longer periods. We demonstrate the wealth of information provided in the time domain, by doubling the number of known massive eclipsing binary systems and identifying 189 new candidate early-type Be and 20 Oe stars in the SMC. In addition, we find that ~80% of Be stars are photometrically variable in the OGLE-III time domain and provide evidence that short-term pulsating stars with additional photometric variability are rotating close to their break-up velocity. [ABSTRACT FROM AUTHOR]

The status of our work on binary classical cepheid systems in the Large Magellanic Cloud is presented. We report on results from our follow up of two eclipsing binary cepheids OGLE-LMC-CEP-0227 and OGLE-LMC-CEP-1812. Here we presented for the first time confirmation that a third cepheid OGLE-LMC-CEP-2532 is a true eclipsing binary cepheid with a period of 800 days. Two other very good candidates for eclipsing binaries detected during OGLE-IV survey are also discussed. [ABSTRACT FROM AUTHOR]

We present some results of long-term studies of pulsating stars conducted in the course of the OGLE and Araucaria projects. In particular, very scarce eclipsing binaries containing pulsating stars are discussed. Such systems provide a unique opportunity to improve calibration of the cosmic distance scale and to better calibrate stellar evolutionary models. [ABSTRACT FROM PUBLISHER]