HOT HB STARS IN GLOBULAR CLUSTERS - PHYSICAL PARAMETERS AND CONSEQUENCES FOR THEORY. IV. SDB CANDIDATES IN M 15

Quantitativespectroscopicanalysesoftwofaintblue stars (V=19 m : 5{2 0 m :0) in the globular cluster M15 are pre- sented. Their derived Teff, gravities and absolute magnitudes (Teff=24000K, logg=5.2, MV=4 m :3; Teff=36000K, logg=5.9, MV=4 m :7, respectively) are matched very well by models for the extreme Horizontal Branch (EHB). Both stars are bona- de subdwarf B stars making M15 only the second globular cluster (after NGC6752) for which the existence of sdB stars hasbeenprovenspectroscopically.Whiletheheliumabundance (one tenth solar) of F1-1 is typical for sdB stars, F2-2 surpris- ingly turned out to be a helium rich star, the rst to be reported as a member of a globular cluster. In the eld population of the Milky way such stars are rare (less than 5% of all sdB stars). Fromitsproximitytotheheliummainsequence,itisspeculated thatF2-2maybeanakedheliumcore,i.e.anextremeHorizontal Branch star which lost (almost) all of its hydrogen-rich enve- lope.