Spectroscopic Analysis of Subluminous B Stars in Binaries - Four   Candidate Systems with Neutron Star/Black Hole Companions Discovered
  The masses of compact objects like white dwarfs, neutron stars and black holes are fundamental to astrophysics, but very difficult to measure. We present the results of an analysis of subluminous B (sdB) stars in close binary systems with unseen compact companions to derive their masses and clarify their nature. Radial velocity curves were obtained from time resolved spectroscopy. The atmospheric parameters were determined in a quantitative spectral analysis. With high resolution spectra we were able to measure the projected rotational velocity of the stars with high accuracy. The assumption of orbital synchronization made it possible to constrain inclination angle and companion mass of the binaries. Five invisible companions have masses that are compatible with white dwarfs or late type main sequence stars. But four sdBs have very massive companions like heavy white dwarfs, neutron stars or even black holes. Such a high fraction of massive compact companions can not be explained with current models of binary evolution. 
S. Geier, C. Karl, H. Edelmann, U. Heber, R. Napiwotzki