2012MNRAS.423..320R

Virtually all close compact binary stars are formed through common envelope (CE) evolution. It is generally accepted that during this crucial evolutionary phase a fraction of the orbital energy is used to expel the envelope. However, it is unclear whether additional sources of energy, such as the recombination energy of the envelope, play an important role. Here we report the discovery of the second and third longest orbital period post-common envelope binaries (PCEBs) containing white dwarf (WD) primaries, i.e. SDSS J121130.94-024954.4 (P_orb = 7.818± 0.002 d) and SDSS J222108.45+002927.7 (P_orb = 9.588± 0.002 d), reconstruct their evolutionary history and discuss the implications for the energy budget of CE evolution. We find that, despite their long orbital periods, the evolution of both systems can still be understood without incorporating recombination energy, although at least small contributions of this additional energy seem to be likely. If recombination energy significantly contributes to the ejection of the envelope, more PCEBs with relatively long orbital periods (P_orb ≳ 1-3 d) harbouring massive WDs (M_wd ≳ 0.8M_☉) should exist.