Photometry and spectroscopy of KELT-11

A global analysis of the system shows that the host star is an evolved subgiant star with T_eff_=5370+/-51K, M_*_=1.438_-0.052_^+0.061^M_{Sun}_, R_*_=2.72_-0.17_^+0.21^R_{Sun}_, logg_*_=3.727_-0.046_^+0.040^, and [Fe/H]=0.180+/-0.075. The planet is a low-mass gas giant in a P=4.736529+/-0.00006 day orbit, with M_P_=0.195+/-0.018M_J_, R_P_=1.37_-0.12_^+0.15^R_J_, {rho}_P_=0.093_-0.024_^+0.028^g/cm^3^, surface gravity logg_P_=2.407_-0.086_^+0.080^, and equilibrium temperature T_eq_=1712_-46_^+51^K. KELT-11 is the brightest known transiting exoplanet host in the southern hemisphere by more than a magnitude and is the sixth brightest transit host to date.

The planet is one of the most inflated planets known, with an exceptionally large atmospheric scale height (2763km), and an associated size of the expected atmospheric transmission signal of 5.6%. These attributes make the KELT-11 system a valuable target for follow-up and atmospheric characterization, and it promises to become one of the benchmark systems for the study of inflated exoplanets.