Lupus-TR-3b: A Low-Mass Transiting Hot Jupiter in the Galactic Plane?

We present a strong case for a transiting hot Jupiter planet identified during a single-field transit survey toward the Lupus Galactic plane. The object, Lupus-TR-3b, transits a V = 17.4 K1 V host star every 3.91405 days. Spectroscopy and stellar colors indicate a host star with effective temperature 5000 ± 150 K, with a stellar mass and radius of 0.87 ± 0.04 M_⊙ and 0.82 ± 0.05 R_⊙, respectively. Limb-darkened transit fitting yields a companion radius of 0.89 +/- 0.07 R_J and an orbital inclination of 88.3_-0.8^+1.3 deg. Magellan 6.5 m MIKE radial velocity measurements reveal a 2.4 σ K = 114 ± 25 m s^-1 sinusoidal variation in phase with the transit ephemeris. The resulting mass is 0.81 ± 0.18 M_J and density 1.4 ± 0.4 g cm^-3. Y-band PANIC image deconvolution reveals a V >= 21 red neighbor 0.4'' away which, although highly unlikely, we cannot conclusively rule out as a blended binary with current data. However, blend simulations show that only the most unusual binary system can reproduce our observations. This object is very likely a planet, detected from a highly efficient observational strategy. Lupus-TR-3b constitutes the faintest ground-based detection to date, and one of the lowest mass hot Jupiters known.