2001PASP..113..937S

We have obtained the optical spectrum (3750-7500 Å) and ultraviolet fluxes at 1521 and 2260 Å for the quasar candidate UITBOC 1574. The optical spectrum shows strong Balmer absorption lines through at least n=9 and the He II line at 4686 Å. We compared the optical spectrum with non-LTE stellar atmosphere models and find T_eff=46,900±3,000 K, logg=5.6±0.3, and helium abundance log(He/H)=-2.0±0.4. We classify the object as a hot subdwarf O star (sdO). With its high effective temperature and low helium abundance, UITBOC 1574 may be considered as belonging to the hot end of the subdwarf B population. The location of this object on the theoretical T_eff-g diagram suggests that it is most likely in a post-extreme horizontal branch evolutionary stage. However, we cannot exclude the possibility that UITBOC 1574 may be a low-mass helium white dwarf precursor. We estimate a spectroscopic distance of 5.6±1.7 kpc with a height of 2.7±0.8 kpc above the Galactic plane if it is a helium-poor sdO. The heliocentric radial velocity of the star is 89±18 km.s^–1.