We have determined a dynamical mass for the companion to HD 33636 which indicates it is a low-mass star instead of an exoplanet . Our result is based on an analysis of Hubble Space Telescope ( HST ) astrometry and ground-based radial velocity data . We have obtained high-cadence radial velocity measurements spanning 1.3 years of HD 33636 with the Hobby-Eberly Telescope at McDonald Observatory . We combined these data with previously published velocities to create a data set that spans nine years . We used this data set to search for , and place mass limits on , the existence of additional companions in the HD 33636 system . Our high-precision astrometric observations of the system with the HST Fine Guidance Sensor 1r span 1.2 years . We simultaneously modeled the radial velocity and astrometry data to determine the parallax , proper motion , and perturbation orbit parameters of HD 33636 . Our derived parallax , \pi _ { abs } = 35.6 \pm 0.2 mas , agrees within the uncertainties with the Hipparcos value . We find a perturbation period P = 2117.3 \pm 0.8 days , semimajor axis a _ { A } = 14.2 \pm 0.2 mas , and system inclination i = 4 \fdg 1 \pm 0 \fdg 1 . Assuming the mass of the primary star M _ { A } = 1.02 \pm 0.03 M _ { \sun } , we obtain a companion mass M _ { B } = 142 \pm 11 M _ { Jup } = 0.14 \pm 0.01 M _ { \sun } . The much larger true mass of the companion relative to its minimum mass estimated from the spectroscopic orbit parameters ( M \sin i = 9.3 M _ { Jup } ) is due to the near face-on orbit orientation . This result demonstrates the value of follow-up astrometric observations to determine the true masses of exoplanet candidates detected with the radial velocity method .