Detached white dwarf + main sequence (WD+MS) systems represent the simplest population of post-common envelope binaries (PCEBs), and their ensemble properties carry important information about common-envelope phase. However, most population synthesis studies do not fully consider the effects of the observational selection biases of the samples used to compare with the theoretical simulations. We present a set of detailed Monte Carlo simulations of the population of WD+MS binaries in the Sloan Digital Sky Survey (SDSS) Data Release 7, which allows us to make a sound comparison with the available observed data. We find that our simulations correctly reproduce the properties of the observed distribution of WD+MS PCEBs. This includes the distribution of orbital periods and of masses of the white dwarf and main sequence stars. These distributions can be correctly reproduced for several choices of the free parameters, although models in which = 10% of the internal energy is used to eject the common envelope, and in which a small common envelope efficiency = 0.3 seem to fit the observational data better. We also find that systems with He-core white dwarfs are over-represented in the observed sample, because of selection effects.

Peer Reviewed