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We present V, Rc photometric observations of the short-period W UMa star V568 Peg. They allowed us to improve its period. The light curve solution revealed that V568 Peg is an overcontact binary of A subtype with moderate fill-out factor. Its components are K stars which undergo partial eclipses. The mass ratio was estimated by q-search analysis. We established existing of big cool spot on the primary component with almost the same parameters during the last 4 years. Based on our light curve solution and the GAIA distance we calculated at the first time the masses, radii and luminosities of the components of V568 Peg., Comment: Accepted for publication in Bulgarian Astronomical Journal

The first observations with the 25 cm telescope of the Shumen Astronomical Observatory led to the following conclusions: (a) Intra-night observations of variable stars with an amplitude larger than 0.1 mag are possible down to 14 mag with an acceptable quality with this setup; (b) The equipment is suitable for observations of bright extended objects with sizes up to 30 arcmin (planets, comets, clusters, nebulae, galaxies) with resolution 0.88 arcsec/pix; (c) The guiding of telescope is very good which makes the equipment appropriate for prolonged patrols; (d) The observations with the 25 cm are already fully remote-controlled; (e) The determined transformation coefficients allow transfer from instrumental to standard photometric system BVRcIc and realization of differential photometry., Comment: Accepted for publication in Bulgarian Astronomical Journal

We present photometric observations in Sloan filters g', i' of the eclipsing W UMa stars USNO-A2.0 1350-17365531, V471 Cas, V479 Lac and V560 Lac. The sinusoidal-like O-C diagram of V471 Cas shows presence of third body with mass of 0.12 Msun (red dwarf) at distance 897 Rsun. The O-C diagram of V479 Lac reveals period decreasing. The results of the light curve solutions are: (i) The targets are overcontact binaries with small fill-out factors; (ii) Their components are F-K stars, comparable in size, whose temperature differences are below 80 K; (iii) All targets undergo partial eclipses and to limit the possible mass ratios we carried out two-step $q$-search analysis. The target global parameters (luminosities, radii, masses) were obtained on the base of their GAIA distances and the results of our light curve solutions. The obtained total mass of V560 Lac turns out smaller than the lower mass limit for the presently existing W UMa binaries of 1.0-1.2 Msun, i.e. this target is an peculiar overcontact system., Comment: accept

Photometric observations of the W UMa binary NSVS 2569022 are presented. The light curve solution reveals that both components are of F spectral type (temperatures $T_1$=$T_2$=6100 K). NSVS 254037 undergoes total eclipse of W subtype and the mass ratio is well-determined. Its exclusive small value of only 0.077 implies that the target probably goes to instability and possible merger. This value ranks NSVS 2569022 in sixth place among the binaries with smallest mass ratio. On the base of empirical relation "period -- total mass" for low mass-ratio binaries we estimated the global parameters of NSVS 2569022: masses $M_1$=1.17 M$_{\odot}$ and $M_2$=0.09 M$_{\odot}$; radii $R_1$=1.19 R$_{\odot}$ and $R_2$=0.38 R$_{\odot}$; luminosities $L_1$=1.73 L$_{\odot}$ and $L_2$=0.17 L$_{\odot}$. An analysis of the characteristics of binaries with extremely low-mass ratios is made. NSVS 2569022 turns out peculiar binary among the W UMa stars with extremely small mass ratios due to its unexpected small fill-out factor of only 0.014 (slightly overcontact configuration)., Comment: Accept

We determined the orbits and stellar parameters of three eccentric eclipsing binaries by light curve solutions of their Kepler data. KIC 8111622 and KIC 10518735 undergo total eclipses while KIC 8196180 reveals partial eclipses. The target components are G and K stars, excluding the primary of KIC 8196180 which is early F star. KIC 8196180 reveals well-visible tidally-induced feature at periastron, i.e. it is an eclipsing heartbeat star. The characteristics of the observed periastron feature (shape, width and amplitude) confirm the theoretical predictions. There are additional out-of-eclipse variations of KIC 8196180 with the orbital period which may be explained by spot activity of synchronously rotating component. Besides worse visible periastron feature KIC 811162 exhibits small amplitude light variations whose period is around 2.3 times shorter than the orbital one. These oscillations were attributed to spot(s) on asynchronously rotating component., Comment: Accepted for publication in Astrophysics and Space Science. arXiv admin note: text overlap with arXiv:1707.02488

Photometric observations in V and I bands and low-dispersion spectra of ten ultrashort-period binaries (NSVS 2175434, NSVS 2607629, NSVS 5038135, NSVS 8040227, NSVS 9747584, NSVS 4876238, ASAS 071829-0336.7, SWASP 074658.62+224448.5, NSVS 2729229, NSVS 10632802) are presented. One of them, NSVS 2729229, is newly discovered target. The results from modeling and analysis of our observations revealed that: (i) Eight targets have overcontact configurations with considerable fillout factor (up to 0.5) while NSVS 4876238 and ASAS 0718-03 have almost contact configurations; (ii) NSVS 4876238 is rare ultrashort-period binary of detached type; (iii) all stellar components are late dwarfs; (iv) the temperature difference of the components of each target does not exceed 400 K; (v) NSVS 2175434 and SWASP 074658.62+224448.5 exhibit total eclipses and their parameters could be assumed as well-determined; (v) NSVS 2729229 shows emission in the H_{alpha} line. Masses, radii and luminosities of the stellar components were estimated by the empirical relation "period, orbital axis" for short- and ultrashort-period binaries. We found linear relations mass-luminosity and mass-radius for the stellar components of our targets., Comment: Accepted for publication in PASA

We determined the orbits and stellar parameters of three eccentric eclipsing binaries by light curve solutions of their Kepler data. KIC 10992733 and KIC 5632781 undergo total eclipses while KIC 10026136 reveals partial eclipses. The components of the targets are G and K stars. KIC 10992733 exhibited variations which were attributed to variable visibility of spot(s) on asynchronously rotating component. KIC 5632781 and KIC 1002613 reveal tidally-induced features at periastron, i.e. they might be considered as eclipsing heartbeat stars. The characteristics of the periastron features (shape, width and amplitude) confirm the theoretical predictions., Comment: 11 pages, 5 figures, accepted for publication in New Astronomy

The eccentricity, periastron angle, orbital inclination, mass ratio, stellar temperatures and relative stellar radii of 12 eclipsing eccentric binaries were determined on the basis of Kepler data. The analysis of their out-of-eclipse variability led to the following results: (i) KIC 10490980 exhibits rotational (spot-type) variability; (ii) Four new heartbeat stars were found: KIC 9344623 and KIC 10296163, which have wide tidally induced light humps and KIC 9119405 and KIC 9673173, which have narrow "W-shape" features; (iii) KIC 4932691 shows oscillations with approximately the 18th harmonic of the orbital period. We established that the eccentric Kepler binaries fall below the envelope P(1-e^2)^{3/2} = 5 days on the period-eccentricity diagram and that there is a surprising lack of eccentric binaries with periods of 25-35 days., Comment: Accepted for publication in Astronomical Journal

We present eleven years of high-resolution, spectroscopic observations for the star EM~Cep. EM~Cep switches between B and Be star states, as revealed by the level of H$\alpha$ emission, but spends most of its time in the B~star state. EM~Cep has been considered to be an eclipsing, near contact binary of nearly equal-massed B stars in order to reproduce regular photometric variations; however, this model is problematic due to the lack of any observed Doppler shift in the spectrum. Our observations confirm that there are no apparent Doppler shifts in the wide spectral lines H$\alpha$ and HeI~6678 in either the B or Be star states.The profiles of HeI~6678 typically exhibited a filled-in absorption core, but we detected weak emission in this line during the highest Be state. Given the lack of observed Doppler shifts, we model EM~Cep as an isolated Be star with a variable circumstellar disk. We can reproduce the observed H$\alpha$ emission profiles over the eleven year period reasonably well with disk masses on the order of $3$ to $10\times\,10^{-11}\;M_*$ in the Be state with the circumstellar disk seen at an inclination of $78^{\circ}$ to the line of sight. From a disk ejection episode in 2014, we estimate a mass loss rate of $\approx\,3\times 10^{-9}\;\rm M_{\odot}\,yr^{-1}$. The derived disk density parameters are typical of those found for the classical Be stars. We therefore suggest that the EM~Cep is a classical Be star and that its photometric variations are the result of $\beta\;$Cep or non-radial pulsations., Comment: 15 pages, 14 figures

We present photometric observations of transit of the exoplanet HAT-P-24b using the Rozhen 2 m telescope. Its solution gives relative stellar radius r_{s}=0.1304 (a/R_{s}=7.669), relative planet radius r_{p}=0.01304 and orbital inclination of 90 degree. The calculated planet radius is R_{p}=1.316 R_{J} and corresponds to planet density of rho_{p}=0.37 g cm^{-3}. Our parameter values are between those of the previous two solutions. We did not find evidences of TTV and TDV signals of HAT-P-24b., Comment: Accepted for publication in Bulgarian Astronomical Journal. 6 pages, 2 figures, 3 tables

We carried out light curve solutions of two eclipsing detached binaries on eccentric orbits observed by Kepler. The orbits and fundamental parameters of KIC 11619964 and KIC 7118545 were determined with a high accuracy by modeling of their photometric data. We found that the temperatures of their components differ by around 2000 K while the radii of their secondaries are more than twice smaller than those of the primaries. We detected a strange "brightening" of KIC 11619964 in the narrow phase range (+-0.0005) around the center of the primary eclipse reaching to 0.018 mag in amplitude. This "mid-eclipse brightening" needs follow-up observations with good time resolution., Comment: 8 pages, 3 figures, 3 tables. Accepted for publication in Bulgarian Astronomical Journal. arXiv admin note: text overlap with arXiv:1506.02920

Photometric observations in V and I bands of six eclipsing binaries at the lower limit of the orbital periods of W UMa stars are presented. Three of them are newly discovered eclipsing systems. The light curve solutions revealed that all short-period targets were contact or overcontact binaries and added new six binaries to the family of short-period systems with estimated parameters. Four binaries have equal in size components and mass ratio near 1. The phase variability of the V-I colors of all targets may be explained by lower temperatures of their back surfaces than those of their side surfaces. Five systems revealed O'Connell effect that was reproduced by cool spots on the side surfaces of their primary components. The light curves of V1067 Her in 2011 and 2012 were fitted by diametrically opposite spots. The applying of the criteria for subdivision of the W UMa stars to our targets led to ambiguous results., Comment: 13 pages, 9 figures, accepted for publication in Research in Astronomy and Astrophysics (RAA)

We carried out light curve solutions of the ultrashort-period binaries with MS components observed by $Kepler$. All six targets turned out almost in thermal contact with contact or slightly overcontact configurations. Two of them, KID 4921906 and KID 6309193, are not eclipsing but reveal ellipsoidal and spot variability. One of the components of KID 8108785 exhibits inherent, quasi-sinusoidal, small-amplitude variability. KID 12055255 turned out a very rare case of ultrashort-period overcontact binary consisting of two M dwarfs. Our modeling indicated that the variability of KID 9532219 is due to eclipses but not to $\delta$ Sct pulsations as it was previously supposed., Comment: 6 pages, 6 figures, 3 tables, accepted for publication in Astron. Nachrichten

We carried out photometric and low-resolution spectral observations of six eclipsing ultrashort-period binaries with MS components. The light curve solutions of the Rozhen observations show that all targets are overcontact systems. We found well-defined empirical relation "period -- semi-major axis" for the short-period binaries and used it for estimation of the global parameters of the targets. Our results revealed that NSVS 925605 is quite interesting target: (a) it is one of a few contact binaries with M components; (b) it exhibits high activity (emission in H$\alpha$ line, X-ray emission, large cool spots, non-Planck energy distribution); (c) its components differ in temperature by 700 K. All appearances of high magnetic activity and huge fillout factor (0.7) of NSVS 925605 might be assumed as a precursor of the predicted merging of close magnetic binaries. Another unusual binary is NSVS 2700153 which reveals considerable long-term variability., Comment: 9 pages, 11 figures, 9 tables, MNRAS (accepted)

The paper presents a sample of newly detected eclipsing binaries from the public Kepler data. Orbits and fundamental parameters of 20 unknown eclipsing binaries were determined by modeling of their photometric data. Most of them are well-detached, high-eccentric binaries. We established that the target KID8552719 satisfied all widespread criteria for a planetary candidate. Fitting its light curve we obtained radius R_p=0.9 R_Nept, distance to the host star a = 42.58 Rsun = 0.198 AU and equilibrium temperatute T_p= 489 K. These values imply a Neptune-size object out of the habitable zone of the host star., Comment: 16 pages, 8 figures, 3 tables. This study was presented at the 7th National Conference of the Bulgarian Astronomical Society, June 08-10, 2012, Dimitrovgrad, Bulgaria. This manuscript will be published in Bulgarian Astronomical Journal, volume 19

We present VRI photometry and low-resolution spectroscopy of the object 2MASS J01074282+4845188. The V-shape of the eclipse, the phase variability of the colour indices as well as the presence of a pre-eclipse hump, standstill and flickering allow us to conclude that it is a nova-like cataclysmic star. This is supported by the observed broad emission Halpha line. Its single profile with a relatively narrow FWHM but large FWZI is typical for a nova-like variable of SW Sex subtype. The observed deep eclipses make the newly discovered cataclysmic star 2MASS J01074282+4845188 an interesting object for future investigation., Comment: 11 pages ; 5 figures; 3 tables (Accepted for publication in New Astronomy)

We propose a procedure for light-curve solution of eclipsing binary stars in the Small Magellanic Cloud for which photometric data have been obtained in the framework of the OGLE project as well as way of determination of the global stellar parameters on the basis of the obtained solutions, some empirical relations as well as the distance to the SMC. Several examples illustrate this procedure., Comment: 10 pages, 2 figures, accepted