HAT-P-13b,c: A transiting hot jupiter with a massive outer companion on an eccentric orbit

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 ± 0.0017days. The outer planet HAT-P-13c orbits the star every P ₂ = 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 _-0.10 M _·, radius of 1.56 ± 0.08R _·, effective temperature of 5653 ± 90K, 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 ± 0.079R _J, yielding a mean density of 0.498^+0.103 _-0.069g cm^-3. The outer companion has m ₂sin i ₂ = 15.2 ± 1.0 M _J, and orbits on a highly eccentric orbit of e ₂ = 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.