We report on a spectroscopic determination of the stellar parameters and chemical abundances for the parent star of the transiting planet TrES-1 .
Based on a detailed analysis of iron lines in our Keck and HET spectra we derive T _ { \mathrm { eff } } = 5250 \pm 75 K , \log g = 4.6 \pm 0.2 , and [ Fe/H ] = 0.00 \pm 0.09 .
By measuring the Ca ii activity indicator and by putting useful upper limits on the Li abundance we constrain the age of TrES-1 to be 2.5 \pm 1.5 Gyr .
By comparing theoretical stellar evolution models with the observational parameters we obtain M _ { \star } = 0.89 \pm 0.05 M _ { \odot } , and R _ { \star } = 0.83 \pm 0.05 R _ { \odot } .
Our improved estimates of the stellar parameters are utilized in a new analysis of the transit photometry of TrES-1 to derive a mass M _ { p } = 0.76 \pm 0.05 M _ { \mathrm { J } } , a radius R _ { p } = 1.04 ^ { +0.08 } _ { -0.05 } R _ { \mathrm { J } } , and an inclination i = 89 ^ { \circ } .5 ^ { +0.5 } _ { -1.3 } .
The improved planetary mass and radius estimates provide the grounds for new crucial tests of theoretical models of evolution and evaporation of irradiated extrasolar giant planets .