Orbital Period Changes and Their Evolutionary Status for the Weak-Contact Binaries. III. AO Camelopardalis and AH Tauri

In this paper, we presented multicolor photometric observations for two eclipsing binaries, AO Camelopardalis and AH Tauri, obtained on 2008 December 16 and 17. Using the Wilson-Devinney Code, the photometric solution of AH Tau was determined from our new CCD data. The mass ratio and the fill-out factor are q = 0.503(+/-0.003) and f = 10.8%(+/-0.1%), respectively. This indicates that AH Tau is in weak contact. For the weak-contact binary AO Cam, BVI light curves clearly show a difference in the heights of the maxima (i.e., the O'Connell effect), which may be explained by spot activity. By analyzing the O - C curves for AO Cam and AH Tau, it is found that the orbital periods appear to show a secular period decrease with a cyclic variation. The observed period modulation is Delta *DP/P ~ 10-6. For AO Cam, the cyclic oscillation with a short period of 7.63(+/-0.07) yr and a low amplitude of 00019(+/-00003) may be preferably attributed to the cyclic magnetic activity. The period and amplitude of the cyclic variation for AH Tau are 45.8(+/-1.1) yr and 00171(+/-00005), which may more likely result from the light-time effect via a third body. The secular period decrease rates are dP/dt = -1.26(+/-0.04) X 10-7 days yr -1 for AO Cam and dP/dt = -6.98(+/-0.07) X 10-8 days yr -1 for AH Tau. This kind of period decrease can be plausibly explained by the mass transfer from the primary to the secondary, and may result in the system evolving into a deep contact configuration.