W UMa systems with marginal contact.

Evidence is presented indicating that W UMa binaries of W-type are undergoing thermal relaxation oscillations about a state of marginal contact. This suggestion receives theoretical support from calculations showing that a newly formed contact binary will evolve on a thermal time scale toward a state of marginal contact and that, if contact is then broken, the system will evolve back into contact, again on a thermal time scale. Observational support comes from earlier light-curve analyses which revealed that marginal contact is a general property of the W-type systems; the instabilities of their light curves and periods also support the contention that they cannot achieve thermal equilibrium. The A-type systems are suggested to be W UMa systems that have been able to achieve thermal equilibrium as a result of nuclear evolution. The relationship of W UMa systems to other shortperiod binaries is also discussed. Subject headings: stars: binaries - stars: W Ursae Majoris

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