  We report homogeneous spectroscopic determinations of the effective
temperature, metallicity, and projected rotational velocity for the host stars
of 56 transiting planets. Our analysis is based primarily on the Stellar
Parameter Classification (SPC) technique. We investigate systematic errors by
examining subsets of the data with two other methods that have often been used
in previous studies (SME and MOOG). The SPC and SME results, both based on
comparisons between synthetic spectra and actual spectra, show strong
correlations between temperature, [Fe/H], and log g when solving for all three
quantities simultaneously. In contrast the MOOG results, based on a more
traditional curve-of-growth approach, show no such correlations. To combat the
correlations and improve the accuracy of the temperatures and metallicities, we
repeat the SPC analysis with a constraint on log g based on the mean stellar
density that can be derived from the analysis of the transit light curves.
Previous studies that have not taken advantage of this constraint have been
subject to systematic errors in the stellar masses and radii of up to 20% and
10%, respectively, which can be larger than other observational uncertainties,
and which also cause systematic errors in the planetary mass and radius.
