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 Tc = 2454779.92979 +/- 0.00038 (BJD) and duration 0.1345 +/- 0.0017 days. 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 +/- 0.08 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 -0.046 M J, and radius of 1.281 +/- 0.079 R J, yielding a mean density of 0.498+0.103 -0.069 g 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.