2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/5993Despite being the most abundant (by space density) of interacting binary stars, W Ursae Majoris (W UMa) stars are not understood structurally. Their stellar components are in physical contact, and share a common convective envelope. Observations demand large-scale energy exchange between components, as do considerations of hydrostatic and thermal equilibrium. Yet solutions in complete equilibrium can account for only a few of the bluest W UMa binaries. Models which permit departures from thermal equilibrium can reproduce the observable properties of later-type W UMa binaries, but they develop relaxation oscillations, during a substantial portion of which the binary components break thermal contact and develop very different effective temperatures, contrary to observational statistics. Massive, early-type contact binaries are also known to exist, but the nature of energy and mass exchange in common radiative envelopes remains virtually unexplored.12 pages, 5 figures, to appear in 3D Stellar Evolution, ed. P.P. Eggleton and D.S.P. Dearborn (ASP Conf. Ser.)Astrophysicstext