HOT SUBLUMINOUS STARS AT HIGH GALACTIC LATITUDES .5. UV SPECTRA OF BINARY AND PECULIAR HOT SUBDWARF STARS

UV spectra observed with the IUE satellite of hat sub-dwarfs showing p eculiarities in optical spectra are presented. Most peculiarities can be explained by the presence of a cool companion star. Assuming that t he UV region is not affected by a cool companion, the fluxes of the ho t component of the binary stars are modelled in the UV using theoretic al atmospheric calculations, thereby separating the fluxes of the cool and hot star. Balmer line profiles in the optical spectra of the hot stars are then corrected for the contribution of the fluxes of the coo l stars in order to perform a precise determination of surface graviti es. From the optical excess fluxes, effective temperatures of the cool companion stars are derived. The values lie within a range of 5750 to 4750 K in T-eff, matching stars of spectral types G2 to K2. Constrain ts on the possible mass and luminosity values for the hot subdwarfs ca n be made. In all cases the cool companions are found to be subgiants (luminosity class IV) rather than main-sequence stars. Based on these investigation we conclude that previous estimates of the binary fracti on as given in the literature likely are in error. Two stars show vari abilities in optical photometry. These objects are candidate eclipsing binaries.