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 \pm 43 ~ { } { K } , \log { g _ { \star } } = 4.07 _ { -0.07 } ^ { +0.04 } and { [ Fe / H ] } = -0.28 \pm 0.04 , with an inferred mass M _ { \star } = 1.09 \pm 0.04 ~ { } { M } _ { \Sun } and radius R _ { \star } = 1.58 _ { -0.09 } ^ { +0.16 } ~ { } { R } _ { \Sun } .
The planetary companion has mass M _ { P } = 0.43 \pm 0.05 ~ { } { M } _ { Jup } , radius R _ { P } = 1.19 _ { -0.08 } ^ { +0.13 } ~ { } { R } _ { Jup } , surface gravity \log { g _ { P } } = 2.86 _ { -0.08 } ^ { +0.06 } , and density \rho _ { P } = 0.31 _ { -0.08 } ^ { +0.07 } ~ { } { g~ { } cm ^ { -3 } } .
The planet is on an orbit with semimajor axis a = 0.079 \pm 0.001 AU and eccentricity e = 0.22 _ { -0.10 } ^ { +0.12 } , which is roughly consistent with circular , and has ephemeris of T _ { c } ( { { BJD _ { TDB } } } ) = 2456347.79679 \pm 0.00036 and P = 7.845631 \pm 0.000046 ~ { } { d } .
Equally plausible fits that employ empirical constraints on the host star parameters rather than isochrones yield a larger planet mass and radius by \sim 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 \sim 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 .