The orbital period distribution of subdwarf-B binaries

Abstract

We present the results of a 3.5 year long campaign to measure orbital periods of subdwarf-B (sdB) star binaries. We directly compare our observed orbital period distribution with that predicted by using binary population synthesis. Up to now, most of our systems seem to have been formed through two of the formation channels discussed by Han et al. (2003, MNRAS 341, p. 669), i.e. the first and the second common envelope ejection (CE) channels. At this point, thanks to the long baseline of our observations, we are starting to detect also very long orbital period systems. These have probably come from a complete different formation path, the first stable Roche Lobe overflow (RLOF) channel in which the first mass transfer phase is stable. This channel is expected to lead to the formation of very wide binaries with typical orbital periods ranging from 1 month to 1 year.