The helium-rich subdwarf CPD-20°1123: A post-common-envelope binary evolving on to the extended horizontal branch

Subluminous B stars come in a variety of flavours including single stars, close and wide binaries, and pulsating and non-pulsating variables. A majority have helium-poor surfaces (helium by number n_He < 1per cent), whilst a minority have extremely helium rich surfaces (n_He > 90per cent). A small number have an intermediate surface helium abundance (≈10-30per cent), accompanied by peculiar abundances of other elements. The questions posed are (i) whether these abundance peculiarities are associated with radiatively driven and time-dependent stratification of elements within the photosphere as the star evolves from a helium-enriched progenitor to become a normal helium-poor sdB star and (ii) whether these phenomena occur only in single sdB stars or are also associated with sdB stars in binaries. We present a fine analysis of the bright intermediate helium sdB star CPD-20°1123 (Albus 1) which shows it to be cool, for a hot subdwarf, with T_eff≈ 23000K and with a surface helium abundance ≈17per cent by number. Other elements do not show extraordinary anomalies; in common with majority sdB stars, carbon and oxygen are substantially depleted, whilst nitrogen is enriched. Magnesium through sulphur appear to be depleted by ≈0.5dex, but chlorine and argon are substantially enhanced. We also present a series of radial velocity measurements which show the star to be a close binary with an orbital period of 2.3d, suggesting it to be a post-common-envelope system. The discovery of an intermediate helium rich sdB star in a close binary in addition to known and apparently single exemplars supports the view that these are very young sdB stars in which radiatively driven stratification of the photosphere is incomplete.