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 sub(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 c 0.10 R sub( )and a mass of 1.30 c 0.10 M sub( ), we estimate the planet radius to be (0.726 c 0.064)R sub(Jup), which implies a mean density of 1.07 -0.30 +0.42 g cm super(-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.