HAT-P-15b: A 10.9 Day Extrasolar Planet Transiting a Solar-type Star

We report the discovery of HAT-P-15b, a transiting extrasolar planet in the 'period valley,' a relatively sparsely populated period regime of the known extrasolar planets. The host star, GSC 2883-01687, is a G5 dwarf with V= 12.16. It has a mass of 1.01 ? 0.04 M , radius of 1.08 ? 0.04 R , effective temperature 5568 ? 90 K, and metallicity [Fe/H] = +0.22 ? 0.08. The planetary companion orbits the star with a period P = 10.863502 ? 0.000027 days, transit epoch Tc = 2454638.56019 ? 0.00048 (BJD), and transit duration 0.2285 ? 0.0015 days. It has a mass of 1.946 ? 0.066 M J and radius of 1.072 ? 0.043 R J yielding a mean density of 1.96 ? 0.22 g cm-3. At an age of 6.8+2.5 --1.6 Gyr, the planet is H/He-dominated and theoretical models require about 2% (10 M {circled plus}) worth of heavy elements to reproduce its measured radius. With an estimated equilibrium temperature of ~820 K during transit, and ~1000 K at occultation, HAT-P-15b is a potential candidate to study moderately cool planetary atmospheres by transmission and occultation spectroscopy.