System Parameters of the Transiting Extrasolar Planet HD 209458b
  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, twelve I-band transits observed between 2001-2003 with the Mt. Laguna Observatory 1m telescope, and six Stromgren b+y transits observed between 2001-2004 with two of the Automatic Photometric Telescopes at Fairborn Observatory. 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 though with improved accuracy. For example, applying robust limits on the stellar mass of 0.93-1.20Msun, we find 1.26 < Rplanet < 1.42 Rjup and 0.59 < Mplanet < 0.70 Mjup. We can reduce the uncertainty on these estimates by including a stellar mass-radius relation constraint, yielding Rplanet = 1.35 +/- 0.07 Rjup and Mplanet = 0.66 +/- 0.04 Mjup. Our results verify that the planetary radius is 10-20% larger than predicted by planet evolution models, confirming the need for an additional mechanism to slow the evolutionary contraction of the planet. A revised ephemeris is derived, T0=2452854.82545 + 3.52474554E (HJD), which now contains an uncertainty in the period of 0.016s and should facilitate future searches for planetary satellites and other bodies in the HD 209458 system. 
Robert A. Wittenmyer, William F. Welsh, Jerome A. Orosz, A.B. Schultz,   W. Kinzel, M. Kochte, F. Bruhweiler, D. Bennum, Gre...