Cepheid Masses:FUSEObservations of S Muscae

S Mus is the Cepheid with the hottest known companion. The large ultravioletflux means that it is the only Cepheid companion for which the velocityamplitude could be measured with the echelle mode of the HST GHRS.Unfortunately, the high temperature is difficult to constrain at wavelengthslonger than 1200 \AA because of the degeneracy between temperature andreddening. We have obtained a FUSE spectrum in order to improve thedetermination of the temperature of the companion. Two regions which aretemperature sensitive near 16,000 K but relatively unaffected by H$_2$absorption (940 \AA, and the Ly $\beta$ wings) have been identified. Bycomparing FUSE spectra of S Mus B with spectra of standard stars, we havedetermined a temperature of 17,000 $\pm$ 500 K. The resultant Cepheid mass is6.0 $\pm$ 0.4 M$_\odot$. This mass is consistent with main sequenceevolutionary tracks with a moderate amount of convective overshoot.Comment: accepted to Ap