  A subset of 51 Hipparcos astrometric binaries among FG dwarfs within 67pc has
been surveyed with the NICI adaptive optics system at Gemini-S, directly
resolving for the first time 17 sub-arcsecond companions and 7 wider ones.
Using these data together with published speckle interferometry of 57 stars, we
compare the statistics of resolved astrometric companions with those of a
simulated binary population. The fraction of resolved companions is slightly
lower than expected from binary statistics. About 10% of astrometric companions
could be "dark" (white dwarfs and close pairs of late M-dwarfs). To our
surprise, several binaries are found with companions too wide to explain the
acceleration. Re-analysis of selected intermediate astrometric data shows that
some acceleration solutions in the original Hipparcos catalog are spurious.
