  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 A flux
excess in the TD-1 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 Teff = 16,420 +- 420 K, logg = 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 Msun, considerably above the
median mass (~0.6 Msun) 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 < Mv < 0.6 for the F7 II star, and 28,970 <
Teff < 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 Alp UMi and
Beta Dor are existing IUE spectra of Cepheids sufficiently deep to rule out the
presence of a white dwarf companion.
