The Faint Object Spectrograph (FOS) aboard Hubble Space Telescope (HST) has been used in the UV to observe the prototypical X-ray pulsar Her X-1 and its companion HZ Her. Optical spectra were also obtained contemporaneously at the Kitt... more
The Faint Object Spectrograph (FOS) aboard Hubble Space Telescope (HST) has been used in the UV to observe the prototypical X-ray pulsar Her X-1 and its companion HZ Her. Optical spectra were also obtained contemporaneously at the Kitt Peak National Observatory (KPNO) 2.1 m. The FOS spectra encompass the 1150-3300 A range near binary orbital phases 0.5 (X-ray maximum) and
After realising that V2368 Oph is a variable star, we started systematic UBV observations of it at Hvar and SPM observatories. A limited set of UBV observations was also obtained by HA at the Turkish National Observatory. Simultaneously... more
After realising that V2368 Oph is a variable star, we started systematic UBV observations of it at Hvar and SPM observatories. A limited set of UBV observations was also obtained by HA at the Turkish National Observatory. Simultaneously with photometric observations, we also begun to collect electronic spectra in Ondrejov, San Pedro Martir, and Dominion Astrophysical Observatory. (6 data files).
New UBV photometry of Algol type binary star U Sge obtained during 1993 are presented, analyzed, and discussed. Wilson-Devinney code was employed to analyze the light curves. Absolute dimensions of the system were obtained, and positions... more
New UBV photometry of Algol type binary star U Sge obtained during 1993 are presented, analyzed, and discussed. Wilson-Devinney code was employed to analyze the light curves. Absolute dimensions of the system were obtained, and positions of both components on H-R diagram were specified. Finally through color curve analysis absolute visual magnitudes were determined.
A detailed analysis of new and existing photometric, spectroscopic and spatial distribution data of the eccentric binary V731 Cep was performed. Spectroscopic orbital elements of the system were obtained by means of cross-correlation... more
A detailed analysis of new and existing photometric, spectroscopic and spatial distribution data of the eccentric binary V731 Cep was performed. Spectroscopic orbital elements of the system were obtained by means of cross-correlation technique. According to the solution of radial velocities with UBVRcIc light curves, V731 Cep consists of two main-sequence stars with masses M$_{1}$=2.577 (0.098) M$_{\odot}$, M$_{2}$=2.017 (0.084) M$_{\odot}$, radii R$_{1}$=1.823 (0.030) R$_{\odot}$, R$_{2}$=1.717 (0.025) R$_{\odot}$, and temperatures T$_{eff1}$=10700 (200) K, T$_{eff2}$=9265 (220) K separated from each other by a=23.27 (0.29) R$_{\odot}$ in an orbit with inclination of 88$^{\circ}$.70 (0.03). Analysis of the O--C residuals yielded a rather long apsidal motion period of U=10000(2500) yr compared to the observational history of the system. The relativistic contribution to the observed rates of apsidal motion for V731 Cep is significant (76%). The combination of the absolute dimensions and the apsidal motion properties of the system yielded consistent observed internal structure parameter (log$\bar{k}_{2,obs}$=-2.36) compared to the theory (log$\bar{k}_{2,theo}$=-2.32). Evolutionary investigation of the binary by two methods (Bayesian and evolutionary tracks) shows that the system is t=133(26) Myr old and has a metallicity of [M/H]=-0.04(0.02) dex. The similarities in the spatial distribution and evolutionary properties of V731 Cep with the nearby ($\rho\sim3^{\circ}$.9) open cluster NGC 7762 suggests that V731 Cep could have been evaporated from NGC 7762.
We present long term ground-based photometric variations of the Algol type binary system SZ Her. Modeling of the system shows that the secondary component is filling Roche lobe. The parameters for the primary and the secondary components... more
We present long term ground-based photometric variations of the Algol type binary system SZ Her. Modeling of the system shows that the secondary component is filling Roche lobe. The parameters for the primary and the secondary components have been determined as M1 ¼ 1:56 0:05 M; M2 ¼ 0:77 0:03 M, R1 ¼ 1:61 0:10 R; R2 ¼ 1:55 0:09 R; L1 ¼ 6:5 0:5 L, L2 ¼ 1:1 0:1 L while the separation of the components is a ¼ 4:9 0:3 R. Newly obtained parameters yield the distance of the system as 302 12 pc. We collected all the photometric and CCD times of mid-eclipse available in the literature and combined them with the newly obtained eight times of light minima. Analysis of the mid-eclipse times indicate a period increase of dP dt ¼ þ3:1ð2Þ 108 days/yr that can be interpreted in terms of the mass transfer rate as dM dt ¼ þ2:62ð3Þ 109 M=yr from the secondary to the primary component. The orbital period of the system oscillates with periods of 118 years and 30 years with corresponding semi-amplitudes of 0.0057 days and 0.0055 days, respectively. These changes were analyzed under the assumption of the existence of third and fourth bodies in the system.
We report the detection of eclipses in GJ 3236, a bright (I = 11.6) very low mass binary system with an orbital period of 0.77 days. Analysis of light- and radial velocity curves of the system yielded component masses of 0.38 +/- 0.02 and... more
We report the detection of eclipses in GJ 3236, a bright (I = 11.6) very low mass binary system with an orbital period of 0.77 days. Analysis of light- and radial velocity curves of the system yielded component masses of 0.38 +/- 0.02 and 0.28 +/- 0.02 Msol. The central values for the stellar radii are larger than the theoretical models predict for these masses, in agreement with the results for existing eclipsing binaries, although the present 5% observational uncertainties limit the significance of the larger radii to approximately 1 sigma. Degeneracies in the light curve models resulting from the unknown configuration of surface spots on the components of GJ 3236 currently dominate the uncertainties in the radii, and could be reduced by obtaining precise, multi-band photometry covering the full orbital period. The system appears to be tidally synchronized and shows signs of high activity levels as expected for such a short orbital period, evidenced by strong Halpha emission lines in the spectra of both components. These observations probe an important region of mass-radius parameter space around the predicted transition to fully-convective stellar interiors, where there are a limited number of precise measurements available in the literature.
Analyses of very accurate CoRoT space photometry, past Johnson V photoelectric photometry and high-resolution echelle spectra led to the determination of improved and consistent fundamental stellar properties of both components of AU... more
Analyses of very accurate CoRoT space photometry, past Johnson V photoelectric photometry and high-resolution echelle spectra led to the determination of improved and consistent fundamental stellar properties of both components of AU Monocerotis. We derived new, accurate ephemerides for both the orbital motion (with a period of ) and the long-term, overall brightness variation (with a period of ) of this strongly interacting Be + G semi-detached binary. It is shown that this long-term variation must be due to attenuation of the total light by some variable circumbinary material. We derived the binary mass ratio MG/MB= 0.17 ± 0.03 based on the assumption that the G-type secondary fills its Roche lobe and rotates synchronously. Using this value of the mass ratio as well as the radial velocities of the G-star, we obtained a consistent light curve model and improved estimates of the stellar masses, radii, luminosities and effective temperatures. We demonstrate that the observed lines of the B-type primary may not be of photospheric origin. We also discover rapid and periodic light changes visible in the high-quality residual CoRoT light curves. AU Mon is put into perspective by a comparison with known binaries exhibiting long-term cyclic light changes.
We present results of a photometric survey for variable stars in the field of the globular cluster Omega Centauri. The observed region was centered roughly on the cluster core and covered 644 arcmin^2. The cluster was monitored on 59... more
We present results of a photometric survey for variable stars in the field of the globular cluster Omega Centauri. The observed region was centered roughly on the cluster core and covered 644 arcmin^2. The cluster was monitored on 59 nights in 1999 and 2000. A total of 117 new variables were identified. Among them there are 16 RR Lyr-type stars, 35 SX Phe variables and 26 eclipsing binaries. A comprehensive catalog including all variable stars so far reported from the cluster field is presented. We list basic photometric properties and provide finding charts for a total of 392 objects. For 313 of them new BV light curves were obtained. The presented sample includes several interesting variables, such as SX Phe stars with extremely short periods of pulsation and several candidates for pulsating K giants. Optical counterparts to 9 X-ray sources detected by XMM and Chandra telescopes were identified: all of them are likely to be foreground variables not related to the cluster.
We present ephemerides and solutions of one Algol-type (KZ Dra) and two overcontact systems (LR Cam and IM Vul) based on V(RI) C CCD observations obtained in the project Prosper (network of amateur observers).
