2017AJ....153..178S

We announce the discovery of KELT-12b, a highly inflated Jupiter-mass planet transiting the mildly evolved, V = 10.64 host star TYC 2619-1057-1. We followed up the initial transit signal in the KELT-North survey data with precise ground-based photometry, high-resolution spectroscopy, precise radial velocity measurements, and high-resolution adaptive optics imaging. Our preferred best-fit model indicates that the host star has T_eff = 6279 ± 51 K, logg_* = 3.89 ± 0.05, [Fe/H] = 0.19_–0.09^+0.08,  M_* = 1.59_–0.09^+0.07 M_☉, and  R_* = 2.37 ± 0.17 R_☉. The planetary companion has  M_P = 0.95 ± 0.14  M_J,  R_P = 1.78_–0.16^+0.17 R_J, logg_P = 2.87_–0.10^+0.09, and density  ρ_P = 0.21_–0.05^+0.07 g cm^–3, making it one of the most inflated giant planets known. Furthermore, for future follow-up, we report a high-precision time of inferior conjunction in BJD_TDB of 2,457,083.660459 ± 0.000894 and period of P=5.0316216±0.000032 days. Despite the relatively large separation of ∼0.07 au implied by its ∼5.03-day orbital period, KELT-12b receives significant flux of 2.38_–0.29^+0.32×10⁹ erg s^–1 cm^–2 from its host. We compare the radii and insolations of transiting gas giant planets around hot (T_eff 6250 K) and cool stars, noting that the observed paucity of known transiting giants around hot stars with low insolation is likely due to selection effects. We underscore the significance of long-term ground-based monitoring of hot stars and space-based targeting of hot stars with the Transiting Exoplanet Survey Satellite to search for inflated gas giants in longer-period orbits.