Detection of the Ultraviolet Spectrum of the Hot Subdwarf Companion of 60 Cygni (B1 Ve) from a Survey of IUE Spectra of Be Stars

We used archival International Ultraviolet Explorer (IUE) high-dispersion, short wavelength spectra data to search for evidence of the spectra of hot subdwarf companions of six rapidly rotating Be stars in binary systems. We searched for the signature of a hot companion through an analysis of the cross-correlation functions of observed and model spectra that were separated into primary and secondary components using a Doppler tomography algorithm and adopted spectroscopic orbital solutions. A positive detection of the flux from a hot companion was made for the reconstructed secondary cross-correlation function of just one target, 60 Cygni (B1 Ve). We estimate that the companion of the Be star in 60 Cygni has $T_{\rm eff} = 42 \pm 4$ kK, mass ratio $M_2/M_1 = 0.15 \pm 0.02$, and monochromatic flux ratio ${f_2}/{f_1} = 0.034 \pm 0.002$ in the spectral region near 1525 \AA. If the companions of the other target Be stars are also hot, then they must be faint and contribute less than $\approx 1\%$ of the UV flux ($<0.6 \%$ in the case of $\gamma$ Cas). We also discuss in an appendix a shell episode of Pleione (28 Tau) recorded in the IUE spectra.