PERIOD VARIATION AND ASYMMETRY LIGHT CURVES OF XY URSAE MAJORIS

摘要

New CCD photometric observations of the chromospherically active binary XY Ursae Majoris (XY UMa) wereobtained every year since 2006. The light curves obtained in the late Spring of 2006 show obvious variations on ashort timescale, while the light curves obtained in 2008 December do not. But both sets of light curves are markedlyasymmetric, and were analyzed using the 2003 version of theWilson–Devinney code with spot model. New absolutephysical parameters are obtained. It is found that the total spotted area on the more massive component covers7 of the photospheric surface in 2008 December. Fitting all available light minimum times including the newlyobtained ones with a sinusoidal ephemeris and a four-part linear ephemeris reveals that the orbital period undergoesquasi-periodic oscillation rather than sinusoidal variations. Between the two mechanisms of magnetic activity anda third body around the eclipsing pair, the former one is more plausible.