KELT-6b: A P ∼ 7.9 day hot saturn transiting a metal-poor star with a long-period companion

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 ^+0.04 _-0.07, and [Fe/H] = -0.28 ± 0.04, with an inferred mass M_· = 1.09 ± 0.04M_· and radius R_· = 1.58^+0.16 _-0.09 R _·. The planetary companion has mass MP = 0.43 ± 0.05M_Jup, radius R_P = 1.19^+0.13 _-0.08 R_Jup, surface gravity log gP = 2.86^+0.06 _-0.08, and density .P = 0.31^+0.07 _-0.08 g cm^-3. The planet is on an orbit with semimajor axis a = 0.079 ± 0.001 AU and eccentricity e = 0.22^+0.12 _-0.10, which is roughly consistent with circular, and has ephemeris of Tc(BJDTDB) = 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.