Orbital Period Changes of Two Very Short-Period W UMa Stars CC Com and V523 Cas: A Possible Evidence for TRO Plus AML Model

Abstract

Orbital period changes of the two very short-period contact binaries, CC Com and V523 Cas, are presented. It is shown that the period of CC Com varies in secular decrease with rate of dP/dt = -4.39 x 10(-8) days/year, while the period of V523 Cas is increasing at rate of dP/dt = +9.09 x 10(-8) days/year. The physical parameters of the two W UMa stars are nearly equal which indicates that the physical properties of the two contact binaries are almost the same. The different types of period changes in the two systems may strongly suggest that, although they are undergoing secular angular momentum loss (AML), the oscillatory character predicted by thermal relaxation oscilation (TRO) theory still exist and they are on the different TRO stages, i.e., CC Com is on the shrinking TRO stage, but V523 Cas is on the expending TRO stage. The present period changes of the two systems can be explained by the combination of the TRO and the AML theories.