UITBOC 1574: A very distant helium-poor subdwarf O star

We have obtained the optical spectrum (3750-7500 Angstrom) and ultraviolet fluxes at 1521 and 2260 Angstrom 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 Angstrom. We compared the optical spectrum with non-LTE stellar atmosphere models and find T-eff = 46,9000 +/-3,000 K, log g = 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 temperat ure 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 t he theoretical T-eff-g diagram suggests that it is most likely in a post-ex treme 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 heli ocentric radial velocity of the star is 89 +/- 18 km s(-1).