Deep, Low Mass Ratio Overcontact Binary Systems. III. CU Tauri and TV Muscae

Abstract

New CCD photometric light curves in the B and V bands of the neglected W UMa - type eclipsing variable star CU Tauri are presented. The O'Connell effect in the V light curve obtained in 2001 by Yang and Liu was about Delta V = +0.015, but it vanished in our 2004 observations. The variations in the levels of bothminima were seen. Our two epochs of light minimum and others compiled from the literature were used for the period study. It is shown that the types of some eclipse times were incorrect and the values of the period obtained by previous investigators were aliases that prevented formation of a plausible O - C curve. A new linear ephemeris was derived, and it is discovered that the orbital period of CU Tau shows a continuous decrease at a rate of dP/dt = - 1.81 x 10(-6) days yr(-1). The present symmetric light curves were solved with the 2003 version of the Wilson-Devinney (W-D) code. Both our solutions and those derived by Yang and Liu reveal that CU Tau is a deep ( f = 50.1% +/- 3.2%), low mass ratio (q = 0.1770 +/- 0.0017) overcontact binary system. Meanwhile, the photoelectric light curves in the B, V, R, and I bands of TV Muscae published by Hilditch and coworkers were reanalyzed with the 2003 version of the W-D code. It is shown that the low mass ratio binary turns out to be a deep overcontact system with f = 74.3% +/- 11.3%. A period analysis with all collected times of light minimum revealed a combination of a long-term period decrease (dP/dt = - 2.16 x 10(-7) days yr(-1)) and a possible cyclic change with a period of 29.1 yr. The rapid long-term period decreases of both systems can be explained as a combination of the mass transfer from the more massive component to the less massive one and the angular momentum loss due to mass outflow from the L2 point. In that way, the overcontact degrees of the two systems will become deeper as their periods decrease, and finally they will evolve into a single rapid-rotation star. However, for CU Tau, the rate of the secular period decrease is very large when compared with the other systems of the same type. This suggests that the long-term period decrease may be part of a long-period periodic change, which we need more data to check.