Stability and energetics of mass transfer in double white dwarfs

The donor star in an interacting double white dwarf lies deep within the po tential well of the accretor, a condition which both alters the minimum mas s transfer rate needed to generate an Eddington luminosity, and limits the fraction of matter which can be driven from the binary in a super-Eddington wind. That mass loss itself alters the threshold condition for the onset o f dynamical instability of the donor star. We show that, unless the mass of the accreting star is extremely near the Chandrasekhar limit, a significan t fraction (exceeding 0.5 for accretors of mass < 1M(.)) must be retained, even when the; nominal accretion luminosity far exceeds the Eddington limit . In super-Eddington accretion, the accreted envelope almost certainly exte nds beyond the orbit of the donor, and will trigger white dwarf merger in a common envelope.