  Employing realistic and consistent atmosphere boundary conditions, we have
generated evolutionary models for brown dwarfs and very-low-mass stars (VLMs)
for different metallicities ([Fe/H]), with and without clouds. We find that the
spread in radius at a given mass and age can be as large as $\sim$10% to
$\sim$25%, with higher-metallicity, higher-cloud-thickness atmospheres
resulting quite naturally in larger radii. For each 0.1 dex increase in [Fe/H],
radii increase by $\sim$1% to $\sim$2.5%, depending upon age and mass. We also
find that, while for smaller masses and older ages brown dwarf radii decrease
with increasing helium fraction ($Y$) (as expected), for more massive brown
dwarfs and a wide range of ages they increase with helium fraction. The
increase in radius in going from $Y=0.25$ to $Y=0.28$ can be as large as
$\sim$0.025 \rj\ ($\sim$2.5%). Furthermore, we find that for VLMs an increase
in atmospheric metallicity from 0.0 to 0.5 dex, increases radii by $\sim$4%,
and from -0.5 to 0.5 dex by $\sim$10%. Therefore, we suggest that opacity due
to higher metallicity might naturally account for the apparent radius anomalies
in some eclipsing VLM systems. Ten to twenty-five percent variations in radius
exceed errors stemming from uncertainities in the equation of state alone. This
serves to emphasize that transit and eclipse measurements of brown dwarf radii
constrain numerous effects collectively, importantly including the atmosphere
and condensate cloud models, and not just the equation of state. At all times,
one is testing a multi-parameter theory, and not a universal radius$-$mass
relation.

Solar and Stellar Astrophysics