Subdwarf B Binaries in the Edinburgh-Cape Survey

We give an update of the results of a campaign to obtain orbital solutions of subdwarf B stars from the Edinburgh-Cape survey (Stobie et al. 1997). To date we have obtained blue spectra of 40 subdwarf B stars from the Edinburgh-Cape catalogue using the grating spectrograph at the 1.9 m Radcliffe telescope at the South African Astronomical Observatory. We find that 17 out of these 40 are certain binaries with a few other objects showing radial velocity variations of small amplitude. The binary fraction found in our sample, after correcting for our binary detection efficiency, is 48%. We have secured the orbital parameters for 4 of the 17 systems and narrowed down the orbits of another 7 to a small range of periods. Out of the four subdwarf B binaries for which we have determined the orbital solution, three have orbital periods that, according to population synthesis studies by Han et al. (2003), suggest they have been formed via a common envelope ejection channel. The masses of the companions, assuming a canonical mass of 0.5 M? for the subdwarf B star, suggest that they are probably white dwarfs. We observed the shortest period binary (3 h) of the three, to search for indications of modulation in the lightcurve due to irradiation of the companion by the subdwarf B star. No indications of reflection effect were found confirming that the companion is indeed a white dwarf. The fourth system with measured orbital parameters shows an orbital period that could correspond to a subdwarf B binary formed either via the common envelope ejection channel or the stable Roche Lobe overflow channel. The aim of the this study: to obtain an independent, statistically significant sample of subdwarf B binaries, with solved orbits, based purely upon the Edinburgh-Cape survey to avoid the uncertain biases of the Palomar-Green and other surveys, is underway