  We report abundance analysis for 6 M giant members of the old open cluster
NGC6791, based on infrared spectroscopy (1.5-1.8 micron) at R=25,000, using the
NIRSPEC spectrograph at the Keck II telescope. We find the iron abundance
<[Fe/H]>= +0.35 +/- 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 <[C/Fe]> = -0.35. Our approach constrains [O/Fe] especially
well, based on the measurement of a number of OH lines near 1.6micron; we find
[O/Fe]=-0.07 +/-0.03. The Solar alpha enhancement is in contrast to the
composition of similar stars in the Galactic bulge. We also find low
12C/13C~10, confirming the presence of extra-mixing processes during the red
giant phase of evolution, up to super solar metallicities.
