Light-curve analysis and orbital period change of the extreme mass-ratio overcontact binary AW CrB

Abstract

Extreme mass-ratio contact binaries with a high degree of overcontact may be in the late evolutionary stages of the contact phase. Detailed photometric analyses and orbital period studies of those systems can provide invaluable information for the coalescence scenario of close binary systems, as recently observed in V1309 Sco. In this paper the first light curve analysis and period study for the totally eclipsing contact binary AW CrB is presented. The VRcIc CCD photometric light curves are analysed by means of the WD code. The asymmetry of the light curves is modelled by a cool star spot on the primary component. It is shown that AW CrB has a high degree of overcontact f=75% and an extreme mass-ratio of q=0.10, placing it among the few contact binaries with the lowest known mass-ratios. The mean density of the primary component suggest that it has evolved away from the ZAMS. Eighty-four times of minimum light are reported, derived from observations available in public archives, the literature, and the observations presented in this paper. The orbital period study shows a continuously increasing period, at a rate of dP/dt=3.58x10^-7 d yr^-1, suggesting a mass transfer from the secondary component to the primary one, implying a further decreasing mass-ratio.