  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 d, transit epoch Tc = 2455304.65122 +- 0.00035 (BJD), and
transit duration 0.1023 +- 0.0010 d. The host star has a mass of 0.82 +- 0.03
Msun, radius of 0.79 + 0.10 - 0.04 Rsun, effective temperature 5079 +- 88 K,
and metallicity [Fe/H] = -0.04 +- 0.08. The planetary companion has a mass of
0.059 +- 0.007 MJ, and radius of 0.565 + 0.072 - 0.032 RJ yielding a mean
density of 0.40 +- 0.10 g 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.
HAT-P-26b is consistent with theoretical models of an irradiated Neptune-mass
planet with a 10 Mearth 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.
