1995A&A...298..577T

UV spectra observed with the IUE satellite of hot subdwarfs showing peculiarities in optical spectra are presented. Most peculiarities can be explained by the presence of a cool companion star. Assuming that the UV region is not affected by a cool companion, the fluxes of the hot component of the binary stars are modelled in the UV using theoretical 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 cool stars in order to perform a precise determination of surface gravities. 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. Constraints on the possible mass and luminosity values for the hot subdwarfs can 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 fraction as given in the literature likely are in error. Two stars show variabilities in optical photometry. These objects are candidate eclipsing binaries.