1997A&A...317L..83M

Quantitative spectroscopic analyses of two faint blue stars T_eff, gravities and absolute magnitudes (T_eff=24000K, logg=5.2, M_V=4.3mag; T_eff=36000K, logg=5.9, M_V=4.7mag, respectively) are matched very well by models for the extreme Horizontal Branch (EHB). Both stars are bona-fide subdwarfs B stars making M15 only the second globular cluster (after NGC6752) for which the existence of sdB stars has been proven spectroscopically. While the helium abundance (one tenth solar) of F1-1 is typical of sdB stars, F2-2 surprisingly turned out to be a helium rich star, the first to be reported as a member of a globular cluster. In the field population of the Milky Way such stars are rare (less than 5% of all sdB stars). From its proximity to the helium main sequence, it is speculated that F2-2 may be a naked helium core, i.e. an extreme Horizontal Branch star which lost (almost) all of its hydrogen-rich envelope.