  We measure dynamical masses for five objects--three ultracool dwarfs, one
low-mass star, and one white dwarf--by fitting orbits to a combination of the
Hipparcos-Gaia Catalog of Accelerations, literature radial velocities, and
relative astrometry. Our approach provides precise masses without any
assumptions about the primary star, even though the observations typically
cover only a small fraction of an orbit. We also perform a uniform re-analysis
of the host stars' ages. Two of our objects, HD 4747B and HR 7672B, already
have precise dynamical masses near the stellar/substellar boundary and are used
to 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 this
companion 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 resolves
its previously noted discrepancy with substellar evolutionary models. HD
68017B, a late-M dwarf, has a mass of $m=0.147\pm 0.003$ $M_\odot$, consistent
with stellar theory and previous empirical estimates based on its absolute
magnitude. The progenitor of the white dwarf Gl 86B has been debated in the
literature, and our dynamical measurement of $m=0.595 \pm 0.010$ $M_\odot$ is
consistent with a higher progenitor mass and younger age for this
planet-hosting binary system. Overall, these case studies represent only five
of the thousands of accelerating systems identified by combining Hipparcos and
Gaia. Our analysis could be repeated for many of them to build a large sample
of companions with dynamical masses.
