  Neutron stars and stellar-mass black holes are the remnants of massive stars,
which ended their lives in supernova explosions. These exotic objects can only
be studied in relatively rare cases. If they are interacting with close
companions they become bright X-ray sources. If they are neutron stars, they
may be detected as pulsars. Only a few hundred such systems are presently known
in the Galaxy. However, there should be many more binaries with basically
invisible compact objects in non-interacting binaries. Here we report the
discovery of unseen compact companions to hot subdwarfs in close binary
systems. Hot subdwarfs are evolved helium-core-burning stars that have lost
most of their hydrogen envelopes, often due to binary interactions. Using
high-resolution spectra and assuming tidal synchronisation of the subdwarfs, we
were able to constrain the companion masses of 32 binaries. While most hot
subdwarf binaries have white-dwarf or late-type main sequence companions, as
predicted by binary evolution models, at least 5% of the observed subdwarfs
must have very massive companions: unusually heavy white dwarfs, neutron stars
and, in some cases, even black holes. We present evolutionary models which show
that such binaries can indeed form if the system has evolved through two
common-envelope phases. This new connection between hot subdwarfs, which are
numerous in the Galaxy, and massive compact objects may lead to a tremendous
increase in the number of known neutron stars and black holes and shed some
light on this dark population and its evolutionary link to the X-ray binary
population.
