  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 one of the ten 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.
