The binary fraction of extreme horizontal branch stars

We have used precise radial velocity measurements of subdwarf-B stars from the Palomar-Green catalogue to look for binary extreme horizontal branch (E HB) stars. We have determined the effective temperature, surface gravity an d surface helium abundance for 20 of the targets from new or existing blue spectra and have compiled published values for these quantities for all but one other. We identify 36 EBB stars in our sample and find that at least 2 1 of these stars are binaries. All but one or two of these are new identifi cations. The minimum binary fraction for EHB stars implied by our survey is 60 +/-8 per cent. Our survey is sensitive to binaries with orbital periods P less than or similar to 10d. For reasonable assumptions concerning the p eriod distribution and the mass ratio distribution of the binaries, we find that the mean detection efficiency of our survey over this range of orbita l periods is 87 per cent. Allowing for this estimated detection efficiency, the fraction of EHB stars that are short-period binaries (0.03d less than or similar to P less than or similar to 10 d) is 69 +/-9 per cent. The valu e is not strongly dependent on the period distribution below P approximate to 10 d or the mean companion mass for these short-period binaries. The orb ital separation of the stars in these binaries is much less than the size o f the red giant from which the EHB star has formed. This is strong evidence that binary star evolution is fundamental to the formation of the majority of EHB stars. If there are also binary EHB stars, the orbital periods of w hich are 10 d, the fraction of EHB stars for which evolution was affected b y the presence of a companion may be much higher, e.g. if one third of EHB stars are binaries with orbital periods 10 d less than or similar to P less than or similar to 100 d, then our observations are consistent with all EH B stars being formed through some type of binary star evolution. We find th at five of the other stars we observed are Likely to be post-EHB stars, one of which is also a binary.