New Data and Improved Parameters for the Extrasolar Transiting Planet OGLE-TR-56b

We report new spectroscopic observations with the Keck HIRES instrument of the recently discovered transiting planet OGLE-TR-56b. Our radial velocity measurements with errors of ~100 m s^-1 show clear variations that are in excellent agreement with the phasing (period and epoch) derived from the OGLE transit photometry. Careful analysis of the spectral line bisectors, along with numerical simulations of blend scenarios, supports the argument for the planetary nature of the companion. The new data, combined with measurements from the previous season, allow an improved determination of the mass of the planet, M_p=(1.45+/-0.23)M_Jup. All available OGLE photometry, including new measurements made during the 2003 season, have also been analyzed to derive an improved value for the planetary radius of R_p=(1.23+/-0.16)R_Jup. We discuss the implications of these results for the theory of extrasolar planets.