Quantitative spectroscopic analyses of two faint blue stars ( V= 19 \hskip { -2.5 pt } \stackrel { m } { . } \hskip { -2.845276 pt } 5 – 20 \hskip { -2.5 pt } \stackrel { m } { . } \hskip { -2.845276 pt } 0 ) in the globular cluster M 15 are presented .
Their derived T _ { eff } , gravities and absolute magnitudes ( T _ { eff } =24000K , log g=5.2 , M _ { V } = 4 \hskip { -2.5 pt } \stackrel { m } { . } \hskip { -2.845276 pt } 3 ; T _ { eff } =36000K , log g=5.9 , M _ { V } = 4 \hskip { -2.5 pt } \stackrel { m } { . } \hskip { -2.845276 pt } 7 , respectively ) are matched very well by models for the Extreme Horizontal Branch ( EHB ) .
Both stars are bona-fide subdwarf B stars making M 15 only the second globular cluster ( after NGC 6752 ) for which the existence of sdB stars has been proven spectroscopically .
While the helium abundance ( one tenth solar ) of F1-1 is typical for 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 .