  The formation of stars above about twenty solar masses and their apparently
high multiplicity remain heavily debated subjects in astrophysics. We have
performed a vast high-resolution radial velocity spectroscopic survey of about
250 O- and 540 B-type stars in the southern Milky Way which indicates that the
majority of stars (> 82%) with masses above 16 solar masses form close binary
systems while this fraction rapidly drops to 20% for stars of 3 solar masses.
The binary fractions of O- type stars among different environment classes are:
clusters (72 \pm 13%), associations (73 \pm 8%), field (43 \pm 13%), and
runaways (69 \pm 11%). The high frequency of close pairs with components of
similar mass argues in favour of a multiplicity originating from the formation
process rather than from a tidal capture in a dense cluster. The high binary
frequency of runaway O stars that we found in our survey (69% compared to
19-26% in previous surveys) points to the importance of ejection from young
star clusters and thus supports the competitive accretion scenario.
