2002A&A...384..908Q

O-C diagrams of two very short-period W-type overcontact binaries, RW Com and CE Leo, are formed by considering the general trends of their O-C curves and the natures of the period changes are analyzed. For RW Com, its period shows a secular decrease with a rate of dP/dt=-6.06x10^–8days/year. Weak evidence indicates that a small-amplitude oscillation is superimposed on the secular decrease. For CE Leo its period is increasing at rate of dP/dt=+7.20x10^–7days/year. The masses of the primary components of the two W UMa stars are nearly the same and the main difference between the two systems is that they have a different mass ratio (q=0.34 for RW Com and 0.50 for CE Leo respectively). The different type of period changes may be the result of their different mass ratio. These findings are in agreement with Qian's (2001MNRAS.328..635Q) conclusion that W-type stars with high-mass ratio (q>0.4) usually show a increasing period, while the periods of low-mass ratio systems (q<0.4) are decreasing. The period oscillation of RW Com may be caused either by the presence of an unseen third body or by magnetic activity cycles. The secular period changes of the two binary stars can be explained by the combination of the thermal relaxation oscillation (TRO) and the variable angular momentum loss (AML) via the change of depth of overcontact.