2006ApJ...636..445C

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^+0.00085_–0.00091)+(2.87598^+0.00012_–0.00017)N (HJD). Assuming that the star has a radius of 1.45±0.10 R_☉and a mass of 1.30±0.10 M_☉, we estimate the planet radius to be (0.726±0.064)R_Jup, which implies a mean density of 1.07^+0.42_–0.30g/cm3. 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.