Deep, low mass ratio overcontact binary systems. IV. V410 Aurigae and XY Bootis

The complete charge-coupled device (CCD) light curves in the V, R, and I bands for the eclipsing binary V410 Aur were observed at the Yunnan Observatory in China. Four new light minimum times of V410 Aur were derived from new observations. The photometric solution for V410 Aur was obtained for the first time, while the orbital elements for XY Boo ( observed by Binnendijk in 1971) were reanalyzed by using the latest Wilson-Devinney code. The third light contributions to the total lights of the two binaries are obtained. The results reveal that both systems are A-subtype W UMa binaries with low mass ratios and deep degrees of overcontact (i.e., q(ph) = 0.1428 and f = 52.4% for V410 Aur, and q(ph) = 0.1855 and f = 55.9% for XY Boo). The photometric mass ratios of the two binaries are very consistent with the spectroscopic mass ratios. By combining the spectroscopic elements with our photometric solutions, the absolute photometric parameters for V410 Aur and XY Boo are ( re) determined. Analyzing the orbital periods of the two binaries, it is discovered that their orbital periods show continuous period increases, suggesting that the systems are undergoing mass transfer from the less massive component to the more massive one through the inner Lagrangian point L-1. This kind of binary, with a low mass ratio, a deep degree of overcontact, and a continuous period increase, may coalesce into a rapidly rotating single star due to tidal instability.