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We measure dynamical masses for five objects--three ultracool dwarfs, onelow-mass star, and one white dwarf--by fitting orbits to a combination of theHipparcos-Gaia Catalog of Accelerations, literature radial velocities, andrelative astrometry. Our approach provides precise masses without anyassumptions about the primary star, even though the observations typicallycover only a small fraction of an orbit. We also perform a uniform re-analysisof the host stars' ages. Two of our objects, HD 4747B and HR 7672B, alreadyhave precise dynamical masses near the stellar/substellar boundary and are usedto validate our approach. For Gl 758B, we obtain a mass of$m=38.1_{-1.5}^{+1.7}$ $M_{Jup}$, the most precise mass measurement of thiscompanion to date. Gl 758B is the coldest brown dwarf with a dynamical mass,and the combination of our low mass and slightly older host-star age resolvesits previously noted discrepancy with substellar evolutionary models. HD68017B, a late-M dwarf, has a mass of $m=0.147\pm 0.003$ $M_\odot$, consistentwith stellar theory and previous empirical estimates based on its absolutemagnitude. The progenitor of the white dwarf Gl 86B has been debated in theliterature, and our dynamical measurement of $m=0.595 \pm 0.010$ $M_\odot$ isconsistent with a higher progenitor mass and younger age for thisplanet-hosting binary system. Overall, these case studies represent only fiveof the thousands of accelerating systems identified by combining Hipparcos andGaia. Our analysis could be repeated for many of them to build a large sampleof companions with dynamical masses.

Precise Dynamical Masses of Directly Imaged Companions from Relative Astrometry, Radial Velocities, and Hipparcos-Gaia DR2 Accelerations