  Mass transfer in an interacting binary will often strip the mass donor of its
entire envelope and spin up the mass gainer to near critical rotation. The
nearby B-type star Regulus represents a binary in the post-mass transfer stage:
it is a rapid rotator with a very faint companion in a 40 d orbit. Here we
present the results of a search for the spectral features of the stripped-down
star in an extensive set of high S/N and high resolution spectra obtained with
the CFHT/ESPaDOnS and TBL/NARVAL spectrographs. We first determine revised
orbital elements in order to set accurate estimates of the orbital Doppler
shifts at the times of observation. We then calculate cross-correlation
functions of the observed and model spectra, and we search for evidence of the
companion signal in the residuals after removal of the strong primary
component. We detect a weak peak in the co-added residuals that has the
properties expected for a faint pre-white dwarf. We use the dependence of the
peak height and width on assumed secondary velocity semiamplitude to derive the
semiamplitude, which yields masses of $M_1/M_\odot = 3.7 \pm 1.4$ and
$M_2/M_\odot = 0.31 \pm 0.10$ (assuming orbital inclination equals the spin
inclination of Regulus). We estimate the pre-white dwarf temperature $T_{\rm
eff} = (20 \pm 4)$~kK through tests with differing temperature model spectra,
and we find the radius $R_2/R_\odot = 0.061 \pm 0.011$ from the component
temperatures and the flux ratio associated with the amplitude of the signal in
the cross-correlation residuals.
