1995A&A...294..497K

Contact binaries in thermal equilibrium do not satisfy the Roche equipotential condition but a more general surface condition which is based on the Roche potential and Bernoulli's equation. Internal mass motions are important particularly in the primary's envelope. This implies that the primary's radius is smaller than predicted by Roche geometry. The structure equations for stationary contact binaries (with spherically averaged components) are derived. They involve averaged properties of the velocity field of the internal mass motions. In late-type systems these properties can be approximately described by few parameters. In the simplest treatment only two parameters ω₁,c₁ are sufficient, where ω₁ expresses the mean kinetic energy of the internal mass motions in the primary's envelope and the coefficient c₁ expresses the correlation of these motions with the orbital motion. For given correlation, ω₁ is roughly determined by the contact condition. The correlation coefficient c₁ can be determined from observations. A first test concerns the system AB And as observed by Hrivnak (1988ApJ...335..319H). The results support the assumption of thermal equilibrium. If AB And is unevolved and in thermal equilibrium, the correlation is small. This is compatible with a velocity field as proposed by Webbink (1977ApJ...215..851W). Observational support for the assumption of thermal equilibrium in late-type systems comes from the high level of magnetic activity and the from the W-type syndrome. Thermal equilibrium explains these facts.