We derive improved system parameters for the HD 209458 system using a model that simultaneously fits both photometric transit and radial velocity observations. The photometry consists of previous Hubble Space Telescope STIS and FGS observations, The radial velocities were derived from Keck HIRES observations. The model properly treats the orbital dynamics of the system and thus yields robust and physically self-consistent solutions. Our set of system parameters agrees with previously published results, although with improved accuracy. For example, applying robust limits on the stellar mass of 0.93-1.20 M , we find 1:26R J < R planet < 1:42R J and 0:59M J < M planet < 0:70M J . We can reduce the uncertainty of these estimates by including a stellar mass-radius relation constraint, yielding R planet ¼ 1:35 AE 0:07 ð Þ R J and M planet ¼ 0:66 AE 0:04 ð Þ M J . Our results verify that the planetary radius is 10%-20% larger than predicted by irradiated planet evolution models, confirming the need for an additional mechanism to slow the evolutionary contraction of the planet. A revised ephemeris is derived, T 0 ¼ 2; 452; 854:82545 þ 3:52474554E ( HJD), which now contains an uncertainty in the period of 0.016 s and should facilitate future searches for planetary satellites and other bodies in the HD 209458 system. Subject headingg s: planetary systems -stars: individual ( HD 209458)

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Abstract. We present the data, and modeling and analysis results from the photometric monitoring of five planetary transits of HD 209458 using the Fine Guidance Sensors (FGS) onboard the Hubble Space Telescope (HST). We have now included the output from all four FGS photometers in our data reduction and analysis increasing our S/N over our previous results. We have modeled the transits as an opaque spherical planet in a circular orbit about a limb darkened spherical star and simultaneously fit the model to the FGS data and published STIS transit data (Brown et al., 2001). The measured light curves show small features, a fraction of the transit depth. Some of these faint bumps and ripples appear to be real. We present an analysis of the FGS transit light curves, showing the results of the model fitting, a search for a possible planetary satellite, and an indication of which small features in the light curve we feel are not instrumental noise.

We present preliminary analysis of new HST observations of the transiting extrasolar planet HD 209458b. Photometric observations were obtained with the Fine Guidance Sensor (FGS) on the Hubble Space Telescope (HST), providing milli-mag precision and high time resolution (40 Hz). The FGS photometry allows us to derive precise stellar/orbital parameters (ephemeris, inclination, limb darkening) and planetary radius, and also allows a search for the presence of planetary rings and satellites. We discuss preliminary results and two approaches to modelling the observations.