Abstract
In this paper we pursue the suggestion by Shu, Lubow, & Anderson and Wang that contact discontinuity (DSC) may exist in the secondary in the expansion thermal relaxation oscillation (TRO) state. It is demonstrated that there is a mass exchange instability in some range of mass ratio for the two components. We show that the assumption of constant volume of the secondary should be relaxed in DSC model. For all mass ratios, the secondary always satisfies the condition that no mass flow returns to the primary through the inner Lagrangian point. The secondary will expand to equilibrate the interaction between the common convective envelope and the secondary. The DSC in contact binary undergoing thermal relaxation does not violate the second law of thermodynamics. The maintaining condition of DSC is derived in the time-dependent model. It is desired to improve the TRO model with the advanced DSC layer in future detailed calculations.
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