  Interacting double white dwarf binaries can give rise to a wide variety of
astrophysical outcomes ranging from faint thermonuclear and Type Ia supernovae
to the formation of neutron stars and stably accreting AM Canum Venaticorum
systems. One key factor affecting the final outcome is whether mass transfer
remains dynamically stable or instead diverges, leading to the tidal disruption
of the donor and the merger of the binary. It is typically thought that for low
ratios of the donor mass to the accretor mass, mass transfer remains stable,
especially if accretion occurs via a disk. In this Letter, we examine low mass
ratio double white dwarf binaries and find that the initial phase of
hydrogen-rich mass transfer leads to a classical nova-like outburst on the
accretor. Dynamical friction within the expanding nova shell shrinks the orbit
and causes the mass transfer rate to increase dramatically above the accretor's
Eddington limit, possibly resulting in a binary merger. If the binary survives
the first hydrogen-rich nova outbursts, dynamical friction within the
subsequent helium-powered nova shells pushes the system even more strongly
towards merger. While further calculations are necessary to confirm this
outcome for the entire range of binaries previously thought to be dynamically
stable, it appears likely that most, if not all, interacting double white dwarf
binaries will merge during the course of their evolution.
