Common envelope evolution and double cores of planetary nebulae

The rate at which close binaries consisting of a white dwarf and a main-sequence companion are formed by common envelope (CE) evolution is statistically studied. The distribution of properties like primary mass, secondary mass, and orbital period in post-CE binaries is studied. Results are presented for four cases bracketing reasonable choices of the initial mass ratio distribution and the fraction alpha of the gravitational binding energy between the secondary and the giant core which is available to eject the envelope. The results indicate that interesting constraints on both these parameters can be derived from the observational data on double cores of PN.