  The formation of a double white dwarf binary likely involves a common
envelope (CE) event between a red giant and a white dwarf (WD) during the most
recent episode of Roche lobe overflow mass transfer. We study the role of
recombination energy with hydrodynamic simulations of such stellar
interactions. We find that the recombination energy helps to expel the common
envelope entirely, while if recombination energy is not taken into account, a
significant fraction of the common envelope remains bound. We apply our
numerical methods to constrain the progenitor system for WD 1101+364 -- a
double WD binary that has well-measured mass ratio of $q=0.87\pm0.03$ and an
orbital period of 0.145 days. Our best-fit progenitor for the pre-common
envelope donor is a 1.5 $M_\odot$ red giant.

Solar and Stellar Astrophysics