Abstract. The effects of starspots on the O-C diagrams of eclipsing binaries are investigated, with a special emphasis on contact binaries. It is shown that starspots modulate the O-C differences in a fundamentally different way than real orbital period variations. Migrating starspots can only introduce high frequency, low amplitude disturbances (generally less than 0.01 d), whose characteristics are numerically specified for typical contact binaries. It becomes evident that the observed systematic short timescale variations of the O-C diagrams of active contact binaries (even those like VW Cep) cannot be attributed to starspots. Furthermore, the basic property of non-linear modulation of O-C differences is emphasised. This non-linearity has serious consequences for the widespread traditional method of O-C diagram analysis. This traditional method is found inappropriate even on a fundamental level, and its mathematical and physical inconsistencies are reviewed.

Abstract.From the light curve analysis of several semi-detached close binary systems, the exponent of the gravity-darkening (GDE) for the Roche lobe filling components has been empirically estimated. The analysis, based on Roche geometry, has been made using the latest improved version of our computer programme. The present method of the light-curve analysis enables simultaneous estimation of the systems' parameters and the gravity-darkening exponents. The reliability of the method has been confirmed by its application to the artificial light curves obtained with a priori known parameters. Further tests with real observations have shown that in the case of well defined light curves the parameters of the system and the value of the gravitydarkening exponent can be reliably estimated. This first part of our analysis presents the results for 9 of the examined systems, that could be briefly summarised as follows: 1) For four of the systems, namely: ZZ Cru, RZ Dra, XZ Sgr and W UMi, there is a very good agreement between empirically estimated and theoretically predicted values for purely radiative and convective envelopes. 2) For the rest five, namely: TT Aur, V Pup, TV Cas, LT Her, and VV UMa, the estimated values of the gravity-darkening exponents were deduced to be larger than theory expects for purely radiative or convective envelopes, respectively. Moreover, it is worthwhile to mention that these values -although greater than theoretical predictions-are: a) Smaller compared to those found by others. b) In two cases, (TV Cas & LT Her), they are normal if some kind of solar type activity (i.e. by employing the Roche model involving a spotted area on the surface of the secondary star) is assumed.3) The large values derived for the two early type systems TT Aur & V Pup, are very possibly connected with the effects of rotation laws, and in some degree with the large contribution of the radiative pressure in the total potential, leading by that to considerable deviations of the stellar surfaces from the assumed classical Roche geometry. 4) Finally, in the VV UMa case, the estimated value of the gravity-darkening exponent -although almost double than the expected for stars with convective envelopes -still it is low in comparison with that estimated by others who have analysed the same data. But, since for this particular binary the anomalous GDE is not supported by recent simultaneous uvby observations (Lazaro et al. 2002), the estimated higher value may be due to the old observational material used.

No abstract

In this study we present and re‐analyse the historical, 1889–1998, light curve (LC) of the eclipsing symbiotic binary AR Pav. For the first time, we show that the timing of mid‐points of eclipses observed during a quiescent phase obeys a quadratic ephemeris, with an initial orbital period P0=605.18 d and a rate of period change . We determined a distance to the system of 5.8±1.5 kpc, the mass ratio of the giant to the hot star, MgMh=0.4±0.1, the mass of the giant, Mg=1.8+1/−0.5 M⊙ and its radius, Rg=167±15 R⊙. During quiescence, the LC has characteristic features similar to those observed in cataclysmic variables (CVs). It can be well reproduced by a model of a large accretion disc surrounding the hot star. However, it is probable that the geometry of the transferred material in the Roche lobe of the accretor in AR Pav is different from that of CVs. During active phases the shape of the LC changes considerably. A complex wave‐like variation developed as a function of the orbital phase with an amplitude of ∼1 mag. It is interpreted in terms of a collisionally heated emission region located on the giant surface and arising from the hot star eruption.

This second part of our study concerning gravity-darkening presents the results for 8 semi-detached close binary systems. From the light-curve analysis of these systems the exponent of the gravity-darkening (GDE) for the Roche lobe filling components has been empirically derived. The method used for the light-curve analysis is based on Roche geometry, and enables simultaneous estimation of the systems' parameters and the gravity-darkening exponents. Our analysis is restricted to the black-body approximation which can influence in some degree the parameter estimation. The results of our analysis are: 1) For four of the systems, namely: TX UMa, β Per, AW Cam and TW Cas, there is a very good agreement between empirically estimated and theoretically predicted values for purely convective envelopes. 2) For the AI Dra system, the estimated value of gravity-darkening exponent is greater, and for UX Her, TW And and XZ Pup lesser than corresponding theoretical predictions, but for all mentioned systems the obtained values of the gravity-darkening exponent are quite close to the theoretically expected values. 3) Our analysis has proved generally that with the correction of the previously estimated mass ratios of the components within some of the analysed systems, the theoretical predictions of the gravity-darkening exponents for stars with convective envelopes are highly reliable. The anomalous values of the GDE found in some earlier studies of these systems can be considered as the consequence of the inappropriate method used to estimate the GDE. 4) The empirical estimations of GDE given in Paper I and in the present study indicate that in the light-curve analysis one can apply the recent theoretical predictions of GDE with high confidence for stars with both convective and radiative envelopes.