  The empirical mass-luminosity relation in the Hyades cluster rests on
dynamical mass determinations for five binary systems, of which one is
eclipsing and the other four are visual or interferometric binaries. The last
one was identified and first measured more than 20 years ago. Here we present
dynamical mass measurements for a new binary system in the cluster, 80 Tau,
which is also a visual pair with a much longer orbital period of about 170 yr.
Although we lack the radial-velocity information that has enabled the
individual mass determinations in all of the previous binaries, we show that it
is still possible to derive the component masses for 80 Tau using only
astrometric observations. This is enabled by the accurate proper motion
measurements from the Hipparcos and Gaia missions, which constrain the orbital
acceleration in the plane of the sky. Separate proper motion values from Gaia
for the primary and secondary provide a direct constraint on the mass ratio.
Our mass measurements, M(A) = 1.63 (+0.30/-0.13) M(sun) and M(B) = 1.11
(+0.21/-0.14) M(sun), are consistent with the mass-luminosity relation defined
by the five previously known systems, which in turn is in good agreement with
current models of stellar evolution.
