2009ApJ...707..446B

We report on the discovery of a planetary system with a close-in transiting hot Jupiter on a near circular orbit and a massive outer planet on a highly eccentric orbit. The inner planet, HAT-P-13b, transits the bright V = 10.622 G4 dwarf star GSC 3416-00543 every P = 2.916260±0.000010 days, with transit epoch T_c= 2454779.92979±0.00038 (BJD) and duration 0.1345±0.0017 days. The outer planet HAT-P-13c orbits the star every P₂= 428.5±3.0 days with a nominal transit center (assuming zero impact parameter) of T_2c = 2454870.4±1.8 (BJD) or time of periastron passage T_2,peri= 2454890.05 ±0.48 (BJD). Transits of the outer planet have not been observed, and may not be present. The host star has a mass of 1.22^+0.05_–0.10M_☉, radius of 1.56±0.08 R_☉, effective temperature of 5653±90 K, and is rather metal-rich with [Fe/H] = +0.41±0.08. The inner planetary companion has a mass of 0.853^+0.029_–0.046M_J, and radius of 1.281±0.079 R _J, yielding a mean density of 0.498^+0.103_–0.069 g/cm3. The outer companion has m₂ sin i₂= 15.2±1.0 M_J, and orbits on a highly eccentric orbit of e₂= 0.691±0.018. While we have not detected significant transit timing variations of HAT-P-13b, due to gravitational and light-travel time effects, future observations will constrain the orbital inclination of HAT-P-13c, along with its mutual inclination to HAT-P-13b. The HAT-P-13 (b, c) double-planet system may prove extremely valuable for theoretical studies of the formation and dynamics of planetary systems.