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We present updated parameters for the star HD 209458 and its transiting giantplanet. The stellar angular diameter $\theta$=0.2254$\pm$0.0017 mas is obtainedfrom the average ratio between the absolute flux observed with the Hubble SpaceTelescope and that of the best-fitting Kurucz model atmosphere. This angulardiameter represents an improvement in precision of more than four timescompared to available interferometric determinations. The stellar radius$R_\star$=1.20$\pm$0.05 R$_{\odot}$ is ascertained by combining the angulardiameter with the Hipparcos trigonometric parallax, which is the maincontributor to its uncertainty, and therefore the radius accuracy should besignificantly improved with Gaia's measurements. The radius of the exoplanet$R_\text{p}$=1.41$\pm$0.06 $R_\text{J}$ is derived from the correspondingtransit depth in the light curve and our stellar radius. From the modelfitting, we accurately determine the effective temperature,$T_\text{eff}$=6071$\pm$20 K, which is in perfect agreement with the value of6070$\pm$24 K calculated from the angular diameter and the integrated spectralenergy distribution. We also find precise values from recent Padova Isochrones,such as $R_\star$=1.20$\pm$0.06 R$_{\odot}$ and $T_\text{eff}$=6099$\pm$41 K.We arrive at a consistent picture from these methods and compare the resultswith those from the literature.

Accurate parameters for HD 209458 and its planet from HST spectrophotometry