Are overcontact binaries undergoing thermal relaxation oscillation with variable angular momentum loss?

Orbital period variations of five W-type overcontact binaries, GW Cep, VY Cet, V700 Cyg, EM Lac and AW Vir, are presented based on the analysis of all available times of light minimum. It is discovered that the period of GW Cep is decreasing at a rate of dP/dt = -6.62 10 -8 d yr -1 . For VY Cet and V700 Cyg, a cyclic oscillation is found superimposed on a secular period increase, which can be explained either by the light-time effect of an assumed third body or by magnetic activity cycles. For the other two, EM Lac and AW Vir, the periods show a secular increase. GW Cep is a low mass ratio system with q = 0.37, while the others are high mass ratio systems (q = 0.67, 0.65, 0.63 and 0.76, respectively). The period changes of the five sample stars are in good agreement with Qian's conclusion that low mass ratio overcontact binaries usually show a decreasing period, while the periods of high mass ratio systems are increasing.

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