  Grid stars and reference stars provide the fundamental global and local
astrometric reference frames for observations by the Space Interferometry
Mission. They must therefore be astrometrically stable at the ~1 uas level. I
present simple formulae in closed form to estimate contamination of these
frames by motions due to stellar companions that go undetected in a
radial-velocity (RV) survey of specified precision. The contamination rate
depends almost entirely on the binary-period distribution function and not on
the details of the mass or eccentricity distributions. Screening by a modest RV
survey (sigma_rv=60 m/s) can reduce the fraction of grid stars having
detectable unmodeled accelerations to <<1%. Reference-star selection promises
to be much more challenging, partly because the requirements of astrometric
stability are more severe and partly because the required density of sources is
about 100 times higher, which latter implies that less satisfactory candidates
will have to be accepted. The tools presented here can help design a
reference-star selection strategy, but a full treatment of the problem will
require better knowledge of their companions in the planetary mass range.
