  We present time series photometry for six partial transits of GJ436b obtained
with the Fine Guidance Sensor instrument on the Hubble Space Telescope (HST).
Our analysis of these data yields independent estimates of the host star's
radius R_star = 0.505 +0.029/-0.020 R_sun, and the planet's orbital period P =
2.643882 +0.000060/-0.000058 d, orbital inclination i = 85.80 +0.21/-0.25 deg,
mean central transit time T_c = 2454455.279241 +0.00026/-0.00025 HJD, and
radius R_p = 4.90 +0.45/-0.33 R_earth. The radius we determine for the planet
is larger than the previous findings from analyses of an infrared light curve
obtained with the Spitzer Space Telescope. Although this discrepancy has a 92%
formal significance (1.7 sigma), it might be indicative of systematic errors
that still influence the analyses of even the highest-precision transit light
curves. Comparisons of all the measured radii to theoretical models suggest
that GJ436b has a H/He envelope of ~10% by mass. We also find that the transit
times for GJ436b are constant to within 10 s over the 11 planetary orbits that
the HST data span. However, the ensemble of published values exhibits a
long-term drift and our mean transit time is 128 s later than that expected
from the Spitzer ephemeris. The sparseness of the currently available data
hinders distinguishing between an error in the orbital period or perturbations
arising from an additional object in the system as the cause of the apparent
trend. Assuming the drift is due to an error in the orbital period we obtain an
improved estimate for it of P = 2.643904 +/- 0.000005 d. This value and our
measured transit times will serve as important benchmarks in future studies of
the GJ436 system. (abridged)
