We report new spectroscopic and photometric observations of the parent stars of the recently discovered transiting planets TrES-3 and TrES-4 .
A detailed abundance analysis based on high-resolution spectra yields [ Fe/H ] = -0.19 \pm 0.08 , T _ { \mathrm { eff } } = 5650 \pm 75 K , and \log g = 4.4 \pm 0.1 for TrES-3 , and [ Fe/H ] = +0.14 \pm 0.09 , T _ { \mathrm { eff } } = 6200 \pm 75 K , and \log g = 4.0 \pm 0.1 for TrES-4 .
The accuracy of the effective temperatures is supported by a number of independent consistency checks .
The spectroscopic orbital solution for TrES-3 is improved with our new radial-velocity measurements of that system , as are the light-curve parameters for both systems based on newly acquired photometry for TrES-3 and a reanalysis of existing photometry for TrES-4 .
We have redetermined the stellar parameters taking advantage of the strong constraint provided by the light curves in the form of the normalized separation a / R _ { \star } ( related to the stellar density ) in conjunction with our new temperatures and metallicities .
The masses and radii we derive are M _ { \star } = 0.928 _ { -0.048 } ^ { +0.028 } ~ { } M _ { \sun } , R _ { \star } = 0.829 _ { -0.022 } ^ { +0.015 } ~ { } R _ { \sun } , and M _ { \star } = 1.404 _ { -0.134 } ^ { +0.066 } ~ { } M _ { \sun } , R _ { \star } = 1.846 _ { -0.087 } ^ { +0.096 } ~ { } R _ { \sun } for TrES-3 and TrES-4 , respectively .
With these revised stellar parameters we obtain improved values for the planetary masses and radii .
We find M _ { p } = 1.910 _ { -0.080 } ^ { +0.075 } ~ { } M _ { \mathrm { Jup } } , R _ { p } = 1.336 _ { -0.036 } ^ { +0.031 } ~ { } R _ { \mathrm { Jup } } for TrES-3 , and M _ { p } = 0.925 \pm 0.082 ~ { } M _ { \mathrm { Jup } } , R _ { p } = 1.783 _ { -0.086 } ^ { +0.093 } ~ { } R _ { \mathrm { Jup } } for TrES-4 .
We confirm TrES-4 as the planet with the largest radius among the currently known transiting hot Jupiters .