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We present a spectroscopic investigation of the Be+sdO binary system HR 2142that is based upon large sets of ultraviolet observations from theInternational Ultraviolet Explorer and ground-based H-alpha observations. Wemeasured radial velocities for the Be star component from these spectra, andcomputed a revised orbit. In order to search for the spectral signature of thehot subdwarf, we cross-correlated the short wavelength end of each IUE spectrumwith a model hot star spectrum, and then we used the predicted Doppler shiftsof the subdwarf to shift-and-add all the cross-correlation functions to theframe of the subdwarf. This merged function shows the weak signal from thespectral lines of the hot star, and a best fit is obtained with a mass ratioM_2/M_1 = 0.07 +/- 0.02, companion temperature T_{eff} > 43 +/- 5 kK, projectedrotational velocity V sin i < 30 km/s, and a monochromatic flux ratio near 1170Angstroms of f_2/f_1 > 0.009 +/- 0.001. This hot subdwarf creates a one-armedspiral, tidal wake in the disk of the Be star, and we present a circumbinarydisk model that can explain the occurrence of shell absorption lines by gasenhancements that occur where gas crossing the gap created by the subdwarfstrikes the disk boundaries. The faint companion of HR 2142 may berepresentative of a significant fraction of Be stars with undetected formermass donor companion stars.

The Hot Companion and Circumbinary Disk of the Be Star HR 2142