2012MNRAS.423.3031N

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< 1 per cent), whilst a minority have extremely helium rich surfaces (n_He > 90 per cent). A small number have an intermediate surface helium abundance ( ≈ 10–30 per 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 ≈ 23 000 K and with a surface helium abundance ≈ 17 per 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 ≈ 0.5 dex, 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 2.3 d, 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. Abundance analysis of four members of the metal-rich open cluster NGC 6253