HAT-P-9b: A LOW-DENSITY PLANET TRANSITING A MODERATELY FAINT F STAR

We report the discovery of a planet transiting a moderately faint (V = 12.3 mag) late F star, with an orbital period of 3.92289 +/- 0.00004 days. From the transit light curve and radial velocity measurements, we determine that the radius of the planet is R(p) = 1.40 +/- 0.06 R(Jup) and that the mass is M(p) = 0.78 +/- 0.09 M(Jup). The density of the new planet, rho(p) = 0.35 +/- 0.06 g cm(-3), fits to the low-density tail of the currently known transiting planets. We find that the center of transit is at T(c) = 2454417.9077 +/- 0.0003 (HJD), and the total transit duration is 0.143 +/- 0.004 days. The host star has M(*) = 1.28 +/- 0.13 M(circle dot) and R(*) = 1.32 +/- 0.07 R(circle dot).