After Lucy (1968) introduced the contact-binary model with a common convective envelope, it was envisaged by Hazlehurst & Meyer-Hofmeister (1973) that a sideways flow of convective elements would carry energy from the more luminous star, the primary, to the less luminous star, the secondary, as a result of horizontal pressure variations. Webbink (1977) extended this picture by noting that the interaction between vertical entropy gradients and large-scale smooth circulation currents in the

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... mmon envelope would provide the necessary redistribution of flux. That is, energy is absorbed by the flow during its vertical motion in the primary and is released during its vertical motion in the secondary. Webbink (1977) mentioned two mechanisms by which a large-scale circulation could be generated: (1) the non-spherically symmetric force field due to rotation and tides which will drive an analogue of classical Eddington-Sweet circulation and (2) differential heating of the base of the common envelope. Although these mechanisms are conceptually different, they are not in practice easy to disentangle, and will certainly both be operating in contact binaries.