  We report spectroscopic observations of the star HIP 50796, previously
considered (but later rejected) as a candidate member of the TW Hya
association. Our measurements reveal it to be a single-lined binary with an
orbital period of 570 days and an eccentricity of e = 0.61. The astrometric
signature of this orbit was previously detected by the HIPPARCOS satellite in
the form of curvature in the proper motion components, although the period was
unknown at the time. By combining our radial velocity measurements with the
HIPPARCOS intermediate data (abscissae residuals) we are able to derive the
full three-dimensional orbit, and determine the dynamical mass of the unseen
companion as well as a revised trigonometric parallax that accounts for the
orbital motion. Given our primary mass estimate of 0.73 solar masses (mid-K
dwarf), the companion mass is determined to be 0.89 solar masses, or about 20%
larger than the primary. The likely explanation for the larger mass without any
apparent contribution to the light is that the companion is itself a closer
binary composed of M dwarfs. The near-infrared excess and X-ray emission
displayed by HIP 50796 support this. Our photometric modeling of the excess
leads to a lower limit to the mass ratio of the close binary of about q = 0.8,
and individual masses of 0.44-0.48 solar masses and 0.41-0.44 solar masses. The
new parallax (20.6 +/- 1.9 mas) is significantly smaller than the original
HIPPARCOS value, and more precise.
