  Knowledge of the stellar parameters for the parent stars of transiting
exoplanets is pre-requisite for establishing the planet properties themselves,
and often relies on stellar evolution models. GJ 436, which is orbited by a
transiting Neptune-mass object, presents a difficult case because it is an M
dwarf. Stellar models in this mass regime are not as reliable as for higher
mass stars, and tend to underestimate the radius. Here we use constraints from
published transit light curve solutions for GJ 436 along with other
spectroscopic quantities to show how the models can still be used to infer the
mass and radius accurately, and at the same time allow the radius discrepancy
to be estimated. Similar systems should be found during the upcoming Kepler
mission, and could provide in this way valuable constraints to stellar
evolution models in the lower main sequence. The stellar mass and radius of GJ
436 are M = 0.452 [-0.012,+0.014] M(Sun) and R = 0.464 [-0.011,+0.009] R(Sun),
and the radius is 10% larger than predicted by the standard models, in
agreement with previous results from well studied double-lined eclipsing
binaries. We obtain an improved planet mass and radius of M = 23.17 +/- 0.79
M(Earth) and R = 4.22 [-0.10,+0.09] R(Earth), a density of rho = 1.69
[-0.12,+0.14] g/cm3, and an orbital semimajor axis of a = 0.02872 +/- 0.00027
AU.
