The white-dwarf companions of 56 Persei and HR 3643

We have obtained low-dispersion IUE spectra of the stars 56 Persei (F4 V) and HR 3643 (F7 II), as part of a survey of late-type stars with a 1565 Angstrom flux excess in the TD-I ultraviolet sky survey. The IUE spectrum of each star reveals the presence of a hot white-dwarf companion. We fit the Ly alpha profile and ultraviolet continuum using pure hydrogen models, but the distance of the primary star is also needed to uniquely constrain the white-dwarf parameters. We derive T-eff=16,420+/-420 K, log g=8.46+/-0.2 for the white dwarf companion to 56 Per, using the photometric distance of 30.1+/-2.8 pc. The implied white-dwarf mass is 0.90+/-0.12 M., considerably above the median mass (similar to 0.6 M.) of single white dwarfs. The parameters of the white dwarf in HR 3643 are not well constrained, mainly due to a large uncertainty in the distance. By assuming a reasonable range of gravity for the white dwarf (7.3<log g<9.0), we derive -1.4<M(v)<0.6 for the F7 II star, and 28,970<T-eff<35,990 K for the white dwarf. Prompted by our detection of a white-dwarf companion of a luminous F star, we have examined the IUE archives to assess the upper limits on possible white-dwarf companions to Cepheids. The detection of a Cepheid-white-dwarf binary would provide important insights concerning the most massive progenitors of white dwarfs. Only for the cases of alpha UMi and beta Dor are existing IUE spectra of Cepheids sufficiently deep to rule out the presence of a white-dwarf companion.