  We present the results of a systematic numerical study of the onset of mass
transfer in double degenerate binary systems and its impact on the subsequent
evolution. All investigated systems belong to the regime of direct impact,
unstable mass transfer. In all of the investigated cases, even those considered
unstable by conventional stability analysis, we find a long-lived mass transfer
phase continuing for as many as several dozen orbital periods. This settles a
recent debate sparked by a discrepancy between earlier SPH calculations that
showed disruptions after a few orbital periods and newer grid-based studies in
which mass transfer continued for tens of orbits. As we show that these
binaries can survive at small separation for hundreds of orbital periods, their
associated gravitational wave signal should be included when calculating the
gravitational wave foreground (although expected to below LISA's sensitivity at
these high frequencies). We also show that the inclusion of the entropy
increase associated with shock-heating of the accreted material reduces the
number of orbits a binary survives given the same initial conditions, although
the effect is not as pronounced when using the appropriate initial conditions.
The use of accurate initial conditions and a correct treatment of shock heating
allows for a reliable time evolution of the temperature, density, and angular
momentum, which are important when considering thermonuclear events that may
occur during the mass transfer phase and/or after merger. Our treatment allows
us to accurately identify when surface detonations may occur in the lead-up to
the merger, as well as the properties of final merger products.
