2007ApJ...670L..41K

We describe the discovery of HAT-P-4b, a low-density extrasolar planet transiting BD +36 2593, a V=11.2 mag slightly evolved metal-rich late F star. The planet's orbital period is 3.056536±0.000057 days with a midtransit epoch of 2,454,245.8154±0.0003 (HJD). Based on high-precision photometric and spectroscopic data, and by using transit light curve modeling, spectrum analysis, and evolutionary models, we derive the following planet parameters: M_p=0.68±0.04 M_J, R_p=1.27±0.05 R_J, ρ_p=0.41±0.06 g/cm3, and a=0.0446±0.0012 AU. Because of its relatively large radius, together with its assumed high metallicity (that of its parent star), this planet adds to the theoretical challenges of explaining inflated extrasolar planets.