  S Mus is the Cepheid with the hottest known companion. The large ultraviolet
flux means that it is the only Cepheid companion for which the velocity
amplitude could be measured with the echelle mode of the HST GHRS.
Unfortunately, the high temperature is difficult to constrain at wavelengths
longer than 1200 \AA because of the degeneracy between temperature and
reddening. We have obtained a FUSE spectrum in order to improve the
determination of the temperature of the companion. Two regions which are
temperature sensitive near 16,000 K but relatively unaffected by H$_2$
absorption (940 \AA, and the Ly $\beta$ wings) have been identified. By
comparing FUSE spectra of S Mus B with spectra of standard stars, we have
determined a temperature of 17,000 $\pm$ 500 K. The resultant Cepheid mass is
6.0 $\pm$ 0.4 M$_\odot$. This mass is consistent with main sequence
evolutionary tracks with a moderate amount of convective overshoot.
