2001MNRAS.328..635Q

Orbital period changes of eight W-type contact binaries (TY Boo, BH Cas, AD Cnc, TX Cnc, RZ Com, LS Del, BB Peg and AA UMa) are presented based on the analysis of their O-C curves. It is found that the periods of the five systems TY Boo, TX Cnc, RZ Com, LS Del and AA UMa show secular increase. For BB Peg, its period increase rate has been revised. For AD Cnc, weak evidence also shows that its orbital period is increasing. For the remaining BH Cas, the three times of light minimum given by Agerer & Hubscher indicate that, recently, its period has been increasing. However, the properties of the period need further study. The mass ratios of all the systems are larger than 0.4. The period increases of the systems may suggest that the W-type W UMa stars with high mass ratio (q>0.4) usually show their period increase.

In order to check this conclusion, secular period changes of 30 W-type contact binaries have been collected from the literature. It is found that systems showing period increase usually have a higher mass ratio (q>0.4), and the periods of low-mass ratio systems (q<0.4) are varying in a secular decrease. This strongly suggests that a relation between the orbital period variation and the mass ratio for W-type contact binaries may exist. If the secular period change is caused by conservative mass transfer between the components, this relation may suggest that the evolution of the W-type systems is oscillation around a critical mass ratio (q∼0.4). However this is highly speculative. On the other hand, the relation may be potentially strong observational evidence for Rahunen's conclusion that angular momentum loss (AML) may enable the components of a contact binary to remain in good contact throughout the thermal relaxation oscillation (TRO) cycle. This connection could be explained by the combination of the TRO and the variable AML via the change of depth of contact, which needs further studies observationally and theoretically.