2014AJ....147...39C

We report the discovery of KELT-6b, a mildly inflated Saturn-mass planet transiting a metal-poor host. The initial transit signal was identified in KELT-North survey data, and the planetary nature of the occulter was established using a combination of follow-up photometry, high-resolution imaging, high-resolution spectroscopy, and precise radial velocity measurements. The fiducial model from a global analysis including constraints from isochrones indicates that the V = 10.38 host star (BD+31 2447) is a mildly evolved, late-F star with T_eff= 6102±43 K, log g_* = 4.07_–0.07^+0.04, and [Fe/H] = -0.28±0.04, with an inferred mass M_{sstarf}= 1.09±0.04 M_☉ and radius R_* = 1.58_–0.09^+0.16R_☉. The planetary companion has mass M_P= 0.43±0.05 M_Jup, radius R_p = 1.19_–0.08^+0.13R_jup, surface gravity log g_p = 2.86_0.08^+0.06, and density ρ_p 0.31_–0.08^+0.07g/cm3. The planet is on an orbit with semimajor axis a = 0.079±0.001 AU and eccentricity e = 0.22_–0.1^+0.12, which is roughly consistent with circular, and has ephemeris of T_c(BJD_TDB) = 2456347.79679±0.00036 and P = 7.845631±0.000046 days. Equally plausible fits that employ empirical constraints on the host-star parameters rather than isochrones yield a larger planet mass and radius by ∼4-7. KELT-6b has surface gravity and incident flux similar to HD 209458b, but orbits a host that is more metal poor than HD 209458 by ∼0.3 dex. Thus, the KELT-6 system offers an opportunity to perform a comparative measurement of two similar planets in similar environments around stars of very different metallicities. The precise radial velocity data also reveal an acceleration indicative of a longer-period third body in the system, although the companion is not detected in Keck adaptive optics images.