1995A&A...293L..25V

We present radial velocity measurements of the extremely iron-deficient "post-AGB" stars HR 4049, HD 44179 and HD 52961. The photometric binary period of HR 4049 is confirmed and refined to 429 days, HD 44179 is found to be a binary with a period of 298 days and a large eccentricity, and also for HD 52961 orbital motion is observed. We have thus conclusive evidence that all the known extremely Fe deficient "post-AGB" stars (BD+39 4926, HR 4049, HD 44179, HD 52961 and HD 46703) are binaries with periods in the order of one to a few years. It is clear then that binarity, with a narrow range of orbital periods, is a key aspect for the understanding of the chemical separation and reaccretion process which causes the peculiar abundance patterns. A mass transferring process in which the evolution of the system was affected without destroying it must have taken place in the recent past. With the observed orbital parameters, it is more likely that the stars represent a particular stage in binary evolution rather than being typical post-AGB stars.