We report abundance analysis for 6 M giant members of the old open cluster NGC 6791 , based on infrared spectroscopy ( 1.5 - 1.8 ~ { } \mu m ) at R=25,000 , using the NIRSPEC spectrograph at the Keck II telescope .
We find the iron abundance \langle [ Fe / H ] \rangle = +0.35 \pm 0.02 , confirming the super solar metallicity of this cluster derived from optical medium-high resolution spectroscopy .
We also measure C , O and other alpha element abundances , finding roughly solar [ alpha /Fe ] and \langle [ C / Fe ] \rangle = -0.35 .
Our approach constrains [ O/Fe ] especially well , based on the measurement of a number of OH lines near 1.6 \mu m ; we find [ O/Fe ] = -0.07 \pm 0.03 .
The Solar alpha enhancement is in contrast to the composition of similar stars in the Galactic bulge .
We also find low ^ { 12 } C / ^ { 13 } C \approx 10 , confirming the presence of extra-mixing processes during the red giant phase of evolution , up to super solar metallicities .