  Generic scalar-tensor theories of gravity predict deviations from Newtonian
physics inside astrophysical bodies. In this paper, we point out that low mass
stellar objects, red and brown dwarf stars, are excellent probes of these
theories. We calculate two important and potentially observable quantities: the
radius of brown dwarfs and the minimum mass for hydrogen burning in red dwarfs.
The brown dwarf radius can differ significantly from the GR prediction and
upcoming surveys that probe the mass-radius relation for stars with masses
$<\mathcal{O}(0.1M_\odot)$ have the potential to place new constraints. The
minimum mass for hydrogen burning can be larger than several presently observed
Red Dwarf stars. This places a new and extremely stringent constraint on the
parameters that appear in the effective field theory of dark energy and rules
out several well-studied dark energy models.
