  Using adaptive optics on the Keck II 10-meter telescope on Mauna Kea, we have
surveyed 24 of the nearest young stars known in search of close companions. Our
sample includes members of the MBM 12 and TW Hydrae young associations and the
classical T Tauri binary UY Aurigae in the Taurus star-forming region. We
present relative photometry and accurate astrometry for 10 close multiple
systems. The multiplicity frequency in the TW Hydrae and MBM 12 groups are high
in comparison to other young regions, though the significance of this result is
low because of the small number statistics. We resolve S 18 into a triple
system including a tight 63 mas (projected separation of 17 AU at a distance of
275 pc) binary for the first time, with a hierarchical configuration
reminiscent of VW Chamaeleontis and T Tauri. Another tight binary in our sample
-- TWA 5Aab (54 mas or 3 AU at 55 pc) -- offers the prospect of dynamical mass
measurement using astrometric observations within a few years, and thus could
be important for testing pre-main sequence evolutionary models. Our
observations confirm with 9-sigma confidence that the brown dwarf TWA 5B is
bound to TWA 5A. We find that the flux ratio of UY Aur has changed
dramatically, by more than a magnitude in the H-band, possibly as a result of
variable extinction. With a smaller flux ratio, the system may once again
become detectable as an optical binary, as it was at the time of its discovery
in 1944. Taken together, our results demonstrate that adaptive optics on large
telescopes is a powerful tool for detecting tight companions, and thus
exploring the frequency and configurations of close multiple systems.
