  The extensive timespan of modern radial velocity surveys has made the
discovery of long-period substellar companions more common in recent years,
however measuring the true masses of these objects remains challenging.
Astrometry from the Gaia mission is expected to provide mass measurements for
many of these long-period companions, but this data is not yet available.
However, combining proper motion data from Gaia DR2 and the earlier Hipparcos
mission makes it possible to measure true masses of substellar companions in
favourable cases. In this work, we combine radial velocities with
Hipparcos-Gaia astrometry to measure the true masses of two recently discovered
long-period substellar companion candidates, HD 92987 B and HD 221420 b. In
both cases, we find that the true masses are significantly higher than implied
by radial velocities alone. A $2087 \pm 19$ m s$^{-1}$ astrometric signal
reveals that HD 92987 B is not close to its $17$ $M_J$ minimum mass but is
instead a $0.2562 \pm 0.0045$ $M_\odot$ star viewed at a near-polar orbital
inclination, whereas the $22.9 \pm 2.2$ $M_J$ HD 221420 b can be plausibly
interpreted as a high-mass "super-planet" or a low-mass brown dwarf. With
semi-major axes of $\sim$10 AU both companions are interesting targets for
direct imaging, and HD 221420 b in particular would be a benchmark metal-rich
substellar object if it proves possible to directly detect. Our results
demonstrate the power of Hipparcos-Gaia astrometry for studying long-period
planet and brown dwarf candidates discovered from radial velocity surveys.
