  Observations over the last decade have indicated that a large fraction of the
stars are part of a binary or multiple system. For our understanding of star
formation it is therefore of crucial importance to characterise the outcome of
the star forming process in terms of binary parameters. This thesis describes
the recovery of the primordial binary population in the nearby OB association
Sco OB2. The current binary population in Sco OB2 is first recovered using two
adaptive optics surveys (ADONIS, NAOS-CONICA), a literature study, and a
detailed analysis of the selection effects of visual, spectroscopic, and
astrometric binary surveys. Our results indicate a binary fraction close to
100%. The mass ratio distribution (among A/B primaries) has the form f(q) =
q^-0.4, while random pairing is excluded. The semi-major axis distribution has
the form f(log a) = constant. Due to its youth and low stellar density, the
current binary population of Sco OB2 is very similar to its primordial binary
population. Our study further indicates a small brown dwarf companion frequency
and a small substellar-to-stellar companion frequency among A and B type stars.
These properties, often referred to as the brown dwarf desert, are a natural
result of the mass ratio distribution in Sco OB2. The embryo ejection scenario
is not necessary to explain observations. The brown dwarf desert may be
ascribed to an excess of planetary companions, rather than by a lack of brown
dwarf companions.

Astrophysics