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The Sun-like star HD209458 harbors a close-in giant planet which transitsacross the star's disk, and thus allows an unprecedented access to the basicparameters of the planet, given a certain knowledge of the basic parameters ofthe star, namely its mass and radius. We present theoretical stellar evolutionmodel calculations for HD 209458 and discuss the uncertainties involved inderiving the stellar mass and radius. We derive the mass, M=1.06 Msun, radius,R=1.18 Rsun,and age, t=5.2 Gyr of the star with uncertainties of 10% or more.The dominant sources of uncertainty remain to be the helium abundance estimateand the treatment of convection, even after an optimistic estimate for theeffective temperature of the star. However, we find that in deriving the radiusof the planet, R_p, the relevant stellar model input is the M/R relation, whichruns orthogonal to a degeneracy in the transit light curve solution and greatlyimproves the estimate of R_p. Theoretically the M/R relation has a loweruncertainty than the M and R separately. We estimate the planet radius and massto be R_p =1.42 +0.10/-0.13 R_J and M_p =0.69 +/- 0.02 M_J.Comment: 24 pages, 5 figures, accepted by the Astrophysical Journal; added  correct version of figure 5 and fixed reference

HD 209458: Physical Parameters of the Parent Star and the Transiting Planet