2011ApJ...728..138H

We report the discovery of HAT-P-26b, a transiting extrasolar planet orbiting the moderately bright V = 11.744 K1 dwarf star GSC 0320-01027, with a period P = 4.234516±0.000015 days, transit epoch T_c= 2455304.65122 ±0.00035 (BJD; Barycentric Julian dates throughout the paper are calculated from Coordinated Universal Time (UTC)), and transit duration 0.1023±0.0010 days. The host star has a mass of 0.82±0.03 M_☉, radius of 0.79^+0.10 _–0.04R_☉, effective temperature 5079±88 K, and metallicity [Fe/H] = -0.04±0.08. The planetary companion has a mass of 0.059±0.007 M_J, and radius of 0.565^+0.072_–0.032R_J yielding a mean density of 0.40 ±0.10 g/cm3. HAT-P-26b is the fourth Neptune-mass transiting planet discovered to date. It has a mass that is comparable to those of Neptune and Uranus, and slightly smaller than those of the other transiting Super-Neptunes, but a radius that is ∼65% larger than those of Neptune and Uranus, and also larger than those of the other transiting Super-Neptunes. HAT-P-26b is consistent with theoretical models of an irradiated Neptune-mass planet with a 10 M _⊕ heavy element core that comprises ≳50% of its mass with the remainder contained in a significant hydrogen-helium envelope, though the exact composition is uncertain as there are significant differences between various theoretical models at the Neptune-mass regime. The equatorial declination of the star makes it easily accessible to both Northern and Southern ground-based facilities for follow-up observations.