  Constraining substellar evolutionary models (SSEMs) is particularly difficult
due to a degeneracy between the mass, age, and luminosity of a brown dwarf. In
cases where a brown dwarf is found as a directly imaged companion to a star, as
in HD 4747 and HD 19467, the mass, age, and luminosity of the brown dwarf are
determined independently, making them ideal objects to use to benchmark SSEMs.
Using the Center for High Angular Resolution Astronomy Array, we measured the
angular diameters and calculated the radii of the host stars HD 4747 A and HD
19467 A. After fitting their parameters to the Dartmouth Stellar Evolution
Database, MESA Isochrones and Stellar Tracks, and Yonsei-Yale isochronal
models, we adopt age estimates of $10.74^{+6.75}_{-6.87}$ Gyr for HD 4747 A and
$10.06^{+1.16}_{-0.82}$ Gyr for HD 19467 A. Assuming the brown dwarf companions
HD 4747 B and HD 19467 B have the same ages as their host stars, we show that
many of the SSEMs under-predict bolometric luminosities by $\sim$ 0.75 dex for
HD 4747 B and $\sim 0.5$ dex for HD 19467 B. The discrepancies in luminosity
correspond to over-predictions of the masses by $\sim$ 12\% for HD 4747 B and
$\sim$ 30\% for HD 19467 B. We also show that SSEMs that take into account the
effect of clouds reduce the under-prediction of luminosity to $\sim 0.6$ dex
and the over-prediction of mass to $\sim 8\%$ for HD 4747 B, an L/T transition
object that is cool enough to begin forming clouds. One possible explanation
for the remaining discrepancies is missing physics in the models, such as the
inclusion of metallicity effects.
