We present a detailed spectroscopic study of 93 solar-type stars that are targets of the NASA/ Kepler mission and provide detailed chemical composition of each target .
We find that the overall metallicity is well-represented by Fe lines .
Relative abundances of light elements ( CNO ) and \alpha elements are generally higher for low-metallicity stars .
Our spectroscopic analysis benefits from the accurately measured surface gravity from the asteroseismic analysis of the Kepler light curves .
The \log g parameter is known to better than 0.03 dex and is held fixed in the analysis .
We compare our T _ { eff } determination with a recent colour calibration of V _ { T } - K _ { S } ( TYCHO V magnitude minus 2MASS K _ { S } magnitude ) and find very good agreement and a scatter of only 80 K , showing that for other nearby Kepler targets this index can be used .
The asteroseismic \log g values agree very well with the classical determination using Fe i -Fe ii balance , although we find a small systematic offset of 0.08 dex ( asteroseismic \log g values are lower ) .
The abundance patterns of metals , \alpha elements , and the light elements ( CNO ) show that a simple scaling by [ Fe/H ] is adequate to represent the metallicity of the stars , except for the stars with metallicity below -0.3 , where \alpha -enhancement becomes important .
However , this is only important for a very small fraction of the Kepler sample .
We therefore recommend that a simple scaling with [ Fe/H ] be employed in the asteroseismic analyses of large ensembles of solar-type stars .