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 \pm 0.03 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 .