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 M _ { \sun } ( mid-K dwarf ) , the companion mass is determined to be 0.89 M _ { \sun } , or \sim 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 q \sim 0.8 , and individual masses of 0.44— 0.48 M _ { \sun } and 0.41— 0.44 M _ { \sun } .
The new parallax ( \pi = 20.6 \pm 1.9 mas ) is significantly smaller than the original HIPPARCOS value , and more precise .