Variation in the orbital period of WUMa-type contact systems

The secular variation in the orbital period P-orb is studied as a function of the mass ratio q of the components in a sample of 73 contact systems of class W UMa constructed from a survey of current (1991-2003) published photometric and spectroscopic data. Almost all the W UMa-systems (> 93% of this sample) are found to have a variation in their orbital periods P-orb which alternates in sign independently of their division into A-and Wsubclasses. A statistical study of this sample in terms of the observed characteristics dP(orb)/dt and q showed that on the average the numbers of increases (35 systems) and decreases (33 systems) in the periods are the same, which indicates the existence of flows directed alternately from one component to the other and illustrates the cyclical character of the thermal oscillations. An analysis of the behavior of dP(orb)/dt as a function of the mass interval of the primary component yields a more accurate value for the mass ratio, q approximate to 0.4 divided by 0.45 at which contact binaries are separated into A-and W-subclasses. No correlations were observed between the fill-out factor for the outer contact configuration, the total mass of the contact system, and the mass ratio of the components, on one hand, and the sign of the secular variation in the period. The physical properties and evolutionary features of these systems are discussed.