  The project Massive Unseen Companions to Hot Faint Underluminous Stars from
SDSS (MUCHFUSS) aims at finding hot subdwarf stars with massive compact
companions like massive white dwarfs (M>1.0 M$_\odot$), neutron stars, or
stellar-mass black holes. We present orbital and atmospheric parameters and put
constraints on the nature of the companions of 12 close hot subdwarf B star
(sdB) binaries found in the course of the MUCHFUSS project. The systems show
periods between 0.14 and 7.4 days. Three systems most likely have white dwarf
companions. SDSS J083006.17+475150.3 is likely to be a rare example of a
low-mass helium-core white dwarf. SDSS J095101.28+034757.0 shows an excess in
the infrared that probably originates from a third companion in a wide orbit.
SDSS J113241.58-063652.8 is the first helium deficient sdO star with a
confirmed close companion. This study brings to 142 the number of sdB binaries
with orbital periods of less than 30 days and with measured mass functions. We
present an analysis of the minimum companion mass distribution and show that it
is bimodal. One peak around 0.1 M$_\odot$ corresponds to the low-mass main
sequence and substellar companions. The other peak around 0.4 M$_\odot$
corresponds to the white dwarf companions. The derived masses for the white
dwarf companions are significantly lower than the average mass for single
carbon-oxygen white dwarfs. In a T$_{\rm eff}$-log(g) diagram of sdB+dM
companions, we find signs that the sdB components are more massive than the
rest of the sample. The full sample was compared to the known population of
extremely low-mass white dwarf binaries as well as short-period white dwarfs
with main sequence companions. Both samples show a significantly different
companion mass distribution. We calculate merger timescales and timescales when
the companion will fill its Roche Lobe and the system evolves into a
cataclysmic variable.

Solar and Stellar Astrophysics