  We use the most recent, complete and independent measurements of masses and
radii of white dwarfs in binaries to bound the class of non-trivial modified
gravity theories, viable after GW170817/GRB170817, using its effect on the
mass-radius relation of the stars. We show that the uncertainty in the latest
data is sufficiently small that residual evolutionary effects, most notably the
effect of core composition, finite temperature and envelope structure, must now
accounted for if correct conclusions about the nature of gravity are to be
made. We model corrections resulting from finite temperature and envelopes to a
base Hamada-Salpeter cold equation of state and derive consistent bounds on the
possible modifications of gravity in the stars' interiors, finding that $Y<
0.14$ at 95\% confidence, an improvement of a factor of three with respect to
previous bounds. Finally, our analysis reveals some fundamental degeneracies
between the theory of gravity and the precise chemical makeup of white dwarfs.
