KELT-11b: A Highly Inflated Sub-Saturn Exoplanet Transiting the v = 8 Subgiant HD 93396

We report the discovery of a transiting exoplanet, KELT-11b, orbiting the bright (V = 8.0) subgiant HD 93396. A global analysis of the system shows that the host star is an evolved subgiant star with Teff = 5370 ± 51K M&z.ast; = 1.438-0.052+0.061 &. R∗ =2.72 -0.21+0.17 R. log g∗ = 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 ±0.018 MJ RP = 1.37 0.120.15RJpP = 0.093 -0.024+0.028g cm-3, surface gravity log GP 2.407-0.086+0.080, and equilibrium temperature Teq = -46+51K. 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 (2763 km), 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.