  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.
