2013A&A...557A..74G

We report the discovery of Kepler-77b (alias KOI-127.01), a Saturn-mass transiting planet in a 3.6-day orbit around a metal-rich solar-like star. We combined the publicly available Kepler photometry (quarters 1-13) with high-resolution spectroscopy from the Sandiford at McDonald and FIES at NOT spectrographs. We derived the system parameters via a simultaneous joint fit to the photometric and radial velocity measurements. Our analysis is based on the Bayesian approach and is carried out by sampling the parameter posterior distributions using a Markov chain Monte Carlo simulation. Kepler-77b is a moderately inflated planet with a mass of M_p=0.430±0.032M_Jup, a radius of R_p=0.960±0.016R_Jup, and a bulk density of ρ_p=0.603±0.055g/cm3. It orbits a slowly rotating (P_rot=36±6days) G5V star with M_*=0.95±0.04M_☉, R_*=0.99±0.02R_☉, T_eff=5520±60K, [M/H]=0.20±0.05dex, that has an age of 7.5±2.0Gyr. The lack of detectable planetary occultation with a depth higher than ∼10ppm implies a planet geometric and Bond albedo of A_g≤0.087±0.008 and A_B≤0.058±0.006, respectively, placing Kepler-77b among the gas-giant planets with the lowest albedo known so far. We found neither additional planetary transit signals nor transit-timing variations at a level of ∼0.5min, in accordance with the trend that close-in gas giant planets seem to belong to single-planet systems. The 106 transits observed in short-cadence mode by Kepler for nearly 1.2 years show no detectable signatures of the planet's passage in front of starspots. We explored the implications of the absence of detectable spot-crossing events for the inclination of the stellar spin-axis, the sky-projected spin-orbit obliquity, and the latitude of magnetically active regions.