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Neutron stars and stellar-mass black holes are the remnants of massive stars,which ended their lives in supernova explosions. These exotic objects can onlybe studied in relatively rare cases. If they are interacting with closecompanions they become bright X-ray sources. If they are neutron stars, theymay be detected as pulsars. Only a few hundred such systems are presently knownin the Galaxy. However, there should be many more binaries with basicallyinvisible compact objects in non-interacting binaries. Here we report thediscovery of unseen compact companions to hot subdwarfs in close binarysystems. Hot subdwarfs are evolved helium-core-burning stars that have lostmost of their hydrogen envelopes, often due to binary interactions. Usinghigh-resolution spectra and assuming tidal synchronisation of the subdwarfs, wewere able to constrain the companion masses of 32 binaries. While most hotsubdwarf binaries have white-dwarf or late-type main sequence companions, aspredicted by binary evolution models, at least 5% of the observed subdwarfsmust have very massive companions: unusually heavy white dwarfs, neutron starsand, in some cases, even black holes. We present evolutionary models which showthat such binaries can indeed form if the system has evolved through twocommon-envelope phases. This new connection between hot subdwarfs, which arenumerous in the Galaxy, and massive compact objects may lead to a tremendousincrease in the number of known neutron stars and black holes and shed somelight on this dark population and its evolutionary link to the X-ray binarypopulation.

Hot subdwarf binaries - Masses and nature of their heavy compact companions