Companions to Subdwarf B Stars — Degenerate or Non-Degenerate?

Abstract

We have found many sdB stars which are single-lined spectroscopic binaries with periods of hours or days which show no sign of a companion in their optical spectra. The minimum companion mass is typically ∼ 0.2M_⊙. To distinguish non-degenerate (M-dwarf) companions from degenerate (white dwarf) companions we obtain a lightcurve. If the lightcurve shows a reflection effect the companion is non-degenerate. White dwarfs show no reflection effect because the ‘reflecting area’ is at about 100 times less than a typical M-dwarf. We can rule out non-degenerate companions in lowinclination binaries, the semi-amplitude of the orbit would then require a massive companion which would be seen directly in the optical spectrum. The method is described in detail by Maxted et. al. (2002) in which we presented lightcurves of four short period sdB binaries and found only one had an M-dwarf companion. In this poster we present preliminary results for 6 more short period sdB binaries.