Follow-up photometry of HAT-P-26

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+/-0.03M_{sun}_, radius of 0.79^+0.10^_-0.04_R_{sun}_, effective temperature 5079+/-88K, and metallicity [Fe/H]=-0.04+/-0.08.

The planetary companion has a mass of 0.059+/-0.007M_J_, and radius of 0.565^+0.072^_-0.032_R_J_ yielding a mean density of 0.40+/-0.10g/cm^3^. 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.