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 H \alpha 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 star , and a best fit is obtained with a mass ratio M _ { 2 } / M _ { 1 } = 0.07 \pm 0.02 , companion temperature T _ { eff } \geq 43 \pm 5 kK , projected rotational velocity V \sin i < 30 km s ^ { -1 } , and a monochromatic flux ratio near 1170 Å of f _ { 2 } / f _ { 1 } > 0.009 \pm 0.001 .
This hot subdwarf creates a one-armed spiral , tidal wake in the disk of the Be star , and we present a circumbinary disk model that can explain the occurrence of shell absorption lines by gas enhancements that occur where gas crossing the gap created by the subdwarf strikes the disk boundaries .
The faint companion of HR 2142 may be representative of a significant fraction of Be stars with undetected former mass donor companion stars .