Spectroscopic Analysis of subluminous B Stars in Binaries with compact   Companions
  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. Based on high resolution spectra we were able to measure the projected rotational velocity of the stars with high accuracy. The assumption of orbital synchronization makes it possible to constrain inclination angle and companion mass of the binaries. Five invisible companions have masses that are compatible with that of normal white dwarfs or late type main sequence stars. But four sdBs have very massive companions like heavy white dwarfs > 1 Mo, neutron stars or even black holes. Such a high fraction of massive compact companions is not expected from current models of binary evolution. 
S. Geier, C. Karl, H. Edelmann, U. Heber, R. Napiwotzki