  A total of 28 young nearby stars (ages $\leq 60$,Myr) have been observed in
the K$_{\rm s}$-band with the adaptive optics imager Naos-Conica of the Very
Large Telescope at the Paranal Observatory in Chile. Among the targets are ten
visual binaries and one triple system at distances between 10 and 130 pc, all
previously known. During a first observing epoch a total of 20 faint stellar or
sub-stellar companion-candidates were detected around seven of the targets.
These fields, as well as most of the stellar binaries, were re-observed with
the same instrument during a second epoch, about one year later. We present the
astrometric observations of all binaries. Their analysis revealed that all
stellar binaries are co-moving. In two cases (HD 119022 AB and FG Aqr B/C)
indications for significant orbital motions were found. However, all
sub-stellar companion-candidates turned out to be non-moving background objects
except PZ Tel which is part of this project but whose results were published
elsewhere. Detection limits were determined for all targets, and limiting
masses were derived adopting three different age values; they turn out to be
less than 10 Jupiter masses in most cases, well below the brown dwarf mass
range. The fraction of stellar multiplicity and of the sub-stellar companion
occurrence in the star forming regions in Chamaeleon are compared to the
statistics of our search, and possible reasons for the observed differences are
discussed.
