2011ApJ...726....3N

We present observations of three distinct transits of HD 17156b obtained with the Fine Guidance Sensors on board the Hubble Space Telescope. We analyzed both the transit photometry and previously published radial velocities to find the planet-star radius ratio R_p/R_{sstarf}= 0.07454±0.00035, inclination i = 86.49^+0.24_–0.20deg, and scaled semimajor axis a/R_{sstarf} = 23.19^+0.32_–0.27. This last value translates directly to a mean stellar density determination ρ_{sstarf}= 0.522^+0.021_–0.018 g/cm3. Analysis of asteroseismology observations by the companion paper of Gilliland et al. provides a consistent but significantly refined measurement of ρ_{sstarf} = 0.5308±0.0040. We compare stellar isochrones to this density estimate and find M_{sstarf}= 1.275±0.018 M_☉ and a stellar age of 3.37^+0.20 _–0.47Gyr. Using this estimate of M_{sstarf} and incorporating the density constraint from asteroseismology, we model both the photometry and published radial velocities to estimate the planet radius R_p= 1.0870±0.0066 R_J and the stellar radius R_{sstarf}= 1.5007±0.0076 R_☉. The planet radius is larger than that found in previous studies and consistent with theoretical models of a solar-composition gas giant of the same mass and equilibrium temperature. For the three transits, we determine the times of mid-transit to a precision of 6.2 s, 7.6 s, and 6.9 s, and the transit times for HD 17156 do not show any significant departures from a constant period. The joint analysis of transit photometry and asteroseismology presages similar studies that will be enabled by the NASA Kepler Mission.