Evolutionary status of late-type contact binaries

A new scenario for evolution of contact binaries is presented and discussed. Arguments are given that W UMa-type systems are formed from detached binaries which lose angular momentum via magnetized wind. This takes typically several Gyr. Such an age is sufficient for the initially more massive component to deplete hydrogen in its core and to fill the Roche lobe. As a result, mass transfer occurs with a reversal of the mass ratio. After a fast mass exchange the present primaries of W UMa-type binaries land close to ZAMS whereas the secondaries develop small helium cores, which makes them considerably over-sized relative to ZAMS stars of the same mass. As a result, both components can fulfill the mass-radius relation for contact systems while being in thermal equilibrium. This solves the "Kuiper paradox". Further mass transfer in a contact phase proceeds on a nuclear time scale of the secondary. Hydrogen rich matter is transferred to the primary in the first part of this phase. This keeps the primary close to ZAMS. When the mass of the secondary decreases below the mass of the original hydrogen-burning core, helium-rich matter is transferred to the primary shifting it towards TAMS. This prediction agrees with observations of the position of the components of contact binaries on the mass-radium diagram. The ultimate fate of the binary is coalescence of both components.