Observational tests of theories of contact binaries

Abstract

Two recent theories of the structure of contact binaries are subjected to a wide variety of observational tests. One theory, according to which a W UMa system at or near zero-age cannot achieve thermal equilibrium and so undergoes thermal relaxation oscillations about a state of marginal contact, appears to be in substantial agreement with observational data. In particular, a previously admitted difficulty is to some extent overcome by the discovery of several possible examples of W UMa systems in the broken-contact phase of these oscillations. The second theory, according to which a W UMa system can achieve thermal equilibrium and does so through the development of a contact discontinuity at the inner contact surface of the secondary, appears, on the other hand, to be in substantial disagreement with the observational data. But since several of the disagreements may no longer apply when this theory achieves its final form - evolutionary sequences are at present indeterminate - it should not yet be regarded as contradicted.