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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. In the distribution of projected rotational velocities signs of tidal locking with the companions are visible. By detecting ellipsoidal variations in the lightcurve of an sdB binary we were able to show that subdwarf binaries with orbital periods up to 0.6 d are most likely synchronized. In this case, the inclination angles and companion masses of the binaries can be tightly constrained. Five invisible companions have masses that are compatible with that of normal white dwarfs or late type main sequence stars. However, four sdBs have compact companions massive enough to be heavy white dwarfs (>1 Msun), neutron stars or even black holes. Such a high fraction of massive compact companions is not expected from current models of binary evolution.
It has been suggested that besides stellar companions, substellar objects in close orbits may be able to trigger mass loss in a common envelope phase and form hot subdwarfs. In an ongoing project we search for close substellar companions combining ti
Common envelope (CE) phases in binary systems where the primary star reaches the tip of the red giant branch are discussed as a formation scenario for hot subluminous B-type (sdB) stars. For some of these objects, observations point to very low-mass
The maximum mass of a neutron star has important implications across multiple research fields, including astrophysics, nuclear physics and gravitational wave astronomy. Compact binary millisecond pulsars (with orbital periods shorter than about a day
The origin of ultra-wide massive binaries (orbital separations $10^3-2times 10^5$~AU) and their properties are not well characterized nor understood. Here we use the second Gaia data release to search for wide astrometric companions to Galactic O-B5
Multi-epoch radial velocity measurements of stars can be used to identify stellar, sub-stellar, and planetary-mass companions. Even a small number of observation epochs can be informative about companions, though there can be multiple qualitatively d