THE MINIMUM MASS-RATIO OF W-URSAE-MAJORIS BINARIES

The minimum mass ratio for tidal stability of a contact binary contain ing two unevolved main-sequence stars is calculated to be q(min) simil ar or equal to 0.09 in the case of a mostly radiative primary, and it is higher if an appreciable fraction of the mass lies in a convective envelope. At least one observed system, AW UMa, has a mass ratio just below this value (q = 0.075), implying that, if the system is stable, the primary must be slightly evolved and must have a very shallow conv ective envelope. Contact binaries with mass ratios significantly below that of AW UMa should not be observed, since they are tidally unstabl e and quickly merge into a single, rapidly rotating object, on a times cale similar to 10(3)-10(4) yr.