  The transiting exoplanet HD 149026b is an important case for theories of
planet formation and planetary structure, because the planet's relatively small
size has been interpreted as evidence for a highly metal-enriched composition.
We present observations of 4 transits with the Near Infrared Camera and
Multi-Object Spectrometer on the Hubble Space Telescope, within a wavelength
range of 1.1--2.0 $\mu$m. Analysis of the light curve gives the most precise
estimate yet of the stellar mean density, $\rho_\star =
0.497^{+0.042}_{-0.057}$ g cm$^{-3}$. By requiring agreement between the
observed stellar properties (including $\rho_\star$) and stellar evolutionary
models, we refine the estimate of the stellar radius: $R_\star =
1.541^{+0.046}_{-0.042}$ $R_\sun$. We also find a deeper transit than has been
measured at optical and mid-infrared wavelengths. Taken together, these
findings imply a planetary radius of $R_p = 0.813^{+0.027}_{-0.025}$ $R_{\rm
Jup}$, which is larger than earlier estimates. Models of the planetary interior
still require a metal-enriched composition, although the required degree of
metal enrichment is reduced. It is also possible that the deeper NICMOS transit
is caused by wavelength-dependent absorption by constituents in the planet's
atmosphere, although simple model atmospheres do not predict this effect to be
strong enough to account for the discrepancy. We use the 4 newly-measured
transit times to compute a refined transit ephemeris.
