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The extensive timespan of modern radial velocity surveys has made thediscovery 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 formany of these long-period companions, but this data is not yet available.However, combining proper motion data from Gaia DR2 and the earlier Hipparcosmission makes it possible to measure true masses of substellar companions infavourable cases. In this work, we combine radial velocities withHipparcos-Gaia astrometry to measure the true masses of two recently discoveredlong-period substellar companion candidates, HD 92987 B and HD 221420 b. Inboth cases, we find that the true masses are significantly higher than impliedby radial velocities alone. A $2087 \pm 19$ m s$^{-1}$ astrometric signalreveals that HD 92987 B is not close to its $17$ $M_J$ minimum mass but isinstead a $0.2562 \pm 0.0045$ $M_\odot$ star viewed at a near-polar orbitalinclination, whereas the $22.9 \pm 2.2$ $M_J$ HD 221420 b can be plausiblyinterpreted as a high-mass "super-planet" or a low-mass brown dwarf. Withsemi-major axes of $\sim$10 AU both companions are interesting targets fordirect imaging, and HD 221420 b in particular would be a benchmark metal-richsubstellar object if it proves possible to directly detect. Our resultsdemonstrate the power of Hipparcos-Gaia astrometry for studying long-periodplanet and brown dwarf candidates discovered from radial velocity surveys.

True masses of the long-period companions to HD 92987 and HD 221420 from Hipparcos-Gaia astrometry