There is growing evidence that some Be stars were spun up through mass transfer in a close binary system, leaving the former mass donor star as a hot, stripped-down object. There are five known cases of Be stars with hot subdwarf (sdO) companions that were discovered through International Ultraviolet Explorer (IUE) spectroscopy. Here we expand the search for Be+sdO candidates using archival FUV spectra from IUE. We collected IUE spectra for 264 stars and formed cross-correlation functions (CCFs) with a model spectrum for a hot subdwarf. Twelve new candidate Be+sdO systems were found, and eight of these display radial velocity variations associated with orbital motion. The new plus known Be+sdO systems have Be stars with spectral subtypes of B0 to B3, and the lack of later-type systems is surprising given the large number of cooler B-stars in our sample. We discuss explanations for the observed number and spectral type distribution of the Be+sdO systems, and we argue that there are probably many Be systems with stripped companions that are too faint for detection through our analysis.

The B emission-line stars are rapid rotators that were probably spun up by mass and angular momentum accretion through mass transfer in an interacting binary. Mass transfer will strip the donor star of its envelope to create a small and hot subdwarf remnant. Here we report on Hubble Space Telescope/STIS far-ultraviolet spectroscopy of a sample of Be stars that reveals the presence of the hot sdO companion through the calculation of cross-correlation functions of the observed and model spectra. We clearly detect the spectral signature of the sdO star in 10 of the 13 stars in the sample, and the spectral signals indicate that the sdO stars are hot, relatively faint, and slowly rotating as predicted by models. A comparison of their temperatures and radii with evolutionary tracks indicates that the sdO stars occupy the relatively long-lived, He-core burning stage. Only 1 of the 10 detections was a known binary prior to this investigation, which emphasizes the difficulty of finding such Be+sdO binaries through optical spectroscopy. However, these results and others indicate that many Be stars probably host hot subdwarf companions.

We present a spectroscopic investigation of the Be+sdO binary system HR 2142 that is based upon large sets of ultraviolet observations from the International Ultraviolet Explorer and ground-based Hα observations. We measured radial velocities for the Be star component from these spectra, and computed a revised orbit. In order to search for the spectral signature of the hot subdwarf, we cross-correlated the short wavelength end of each IUE spectrum with a model hot star spectrum, and then we used the predicted Doppler shifts of the subdwarf to shift-and-add all the cross-correlation functions to the frame of the subdwarf. This merged function shows the weak signal from the spectral lines of the hot

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 eff = 42 ± 4 kK, mass ratio M 2 /M 1 = 0.15 ± 0.02, and monochromatic flux ratio f 2 /f 1 = 0.034 ± 0.002 in the spectral region near 1525Å. If the companions of the other target Be stars are also hot, then they must be faint and contribute less than ≈ 1% of the UV flux (< 0.6% in the case of γ Cas). We also discuss in an appendix a shell episode of Pleione (28 Tau) recorded in the IUE spectra.