Hot subluminous stars at high galactic latitudes. IV: Physical parameters and distances of 18 hot subdwarf stars and their spatial distribution

We present Stromgren uvby photometry and optical spectra in the wavelength range of 4000 to 5000 A of 18 hot subdwarf stars; 13 of them are classified as sdB, 4 as sdOB and one as sdO star. Effective temperatures and surface gravities are determined using reddening free colour indices of the Stromgren and the Greenstein system and by fitting theoretical line profiles to the observed Balmer lines. Interpreting the hot subdwarfs as Extended Horizontal-Branch stars, we assume their masses to lie in the small range M = 0.50 ± 0.02M ○ . and calculate their distances. The helium abundances, determined from the equivalent widths of the λ4471 A absorption line, do not show any correlation with the physical parameters