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

We report new spectroscopic observations of the recently discovered transiting planet OGLE-TR-56b with the Keck/HIRES instrument. Our radial velocity measurements with errors of ~100 m/s show clear variations that are in excellent agreement with the phasing (period and epoch) derived from the OGLE transit photometry, confirming 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 this 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.