though still low-mass stars, had to be several times more massive than the WDs they produced. As low-mass giants obey a tight relation between core mass and radius, the mass of the younger WD gives the strongest known constraint on the pre-mass-transfer binary configuration. On the other hand, the initially lower-mass companion must evolve into a white dwarf via a common-envelope process in the last binary interaction phase, in order to achieve the small separations observed [@IL93; @Webbink08]. However, the physics underlying the evolutionary channel(s) by which DWDs form remains uncertain.

A variety of explanations have been posited to explain the first stage of mass transfer (MT), in which the initial primary loses its envelope. [@Han1998] showed that both CE and Roche-lobe overflow (RLOF) are possible in the first episode of the MT. Later, it was claimed that this phase cannot be described by stable and conservative RLOF [@Nelemans00; @Sluys2006]. The suggestion that the first MT episode in the formation history was a CE event also proved problematic: when formulated in terms of the binding energy of the envelope, it appeared to imply that the binary must be able to eject its envelope with an unphysical (often negative) efficiency [@Nelemans00; 