Five new transits of the super-neptune HD 149026b

We present new photometry of HD 149026 spanning five transits of its "super-Neptune" planet. In combination with previous data, we improve on the determination of the planet-to-star radius ratio: R_p/R* = 0.0491_-0.0005+^0.0018. We find the planetary radius to be 0.71 ± 0.05 R_Jup, in accordance with previous theoretical models invoking a high metal abundance for the planet. The limiting error is the uncertainty in the stellar radius. Although we find agreement among four different ways of estimating the stellar radius, the uncertainty remains at 7%. We also present a refined transit ephemeris and a constraint on the orbital eccentricity and argument of pericenter, e cos ω = -0.0014 ± 0.0012, based on the measured interval between primary and secondary transits.