  We use multi-band photometry to refine estimates for the planetary radius and
orbital inclination of the transiting planet system HD 209458. We gathered 1066
spectra over four distinct transits with the STIS spectrometer on the Hubble
Space Telescope using two gratings with a resolution R=1500 and a combined
wavelength range of 290-1030 nm. We divide the spectra into ten
spectrophotometric bandpasses, five for each grating, of equal wavelength span
within each grating, and fit a transit curve over all bandpasses
simultaneously. In our fit we use theoretical values for the stellar
limb-darkening to further constrain the planetary radius. We find that the
radius of HD 209458b is 1.320 +/- 0.025 R_Jup, which is a factor of two more
precise than current estimates. We also obtain improved estimates for the
orbital period P and time of center of transit T_C. Although in principle the
photon-limited precision of the STIS data should allow us to measure the timing
of individual transits to a precision of 2-7 s, we find that systematic
instrumental offsets in the measured flux from one orbit of the spacecraft to
the next degrade these measurements to a typical precision of +/- 14 s. Within
this level of error, we find no significant variations in the timing of the
eight events examined in this work.
