Transit photometry of the core-dominated planet HD 149026b

We report g, V, and r photometric time series of HD 149026 spanning predicted times of transit of the Saturn-mass planetary companion, which was recently discovered by Sato and collaborators. We present a joint analysis of our observations and the previously reported photometry and radial velocities of the central star. We refine the estimate of the transit ephemeris to T-c = (2, 453, 527.87455(-0.00091)(+0.00085)) + (2.87598(-0.00017)(+0.00012))N (HJD). Assuming that the star has a radius of 1: 45 +/- 0.10 R-circle dot and a mass of 1.30 +/- 0.10 M-circle dot, we estimate the planet radius to be (0.726 +/- 0.064)R-Jup, which implies a mean density of 1.07(-0.30)(+0.42) g cm(-3). This density is significantly greater than predicted for models that include the effects of stellar insolation and in which the planet has only a small core of solid material. Thus, we confirm that this planet likely contains a large core and that the ratio of core mass to total planet mass is more akin to that of Uranus and Neptune than to either Jupiter or Saturn.