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Open clusters have long been used to investigate the Galactic disk age-chemical abundance relation. One assumption inherent in these studies is that the surface abundances of open cluster stars are representative of the chemical abundances of the interstellar medium (ISM) from which they formed. However, the bimodal CN inhomogeneities seen in the higher metallicity globular clusters 47~Tuc and M~71 cast doubt on this assumption. NGC~6791 is one of the most globular-cluster-like of the old open clusters in both age ($\sim$10 Gyr) and richness. We have obtained spectra of the 3600--4600 \AA\ wavelength region for twenty red-horizontal-branch (RHB) stars, and have measured indices sensitive to Ca and Fe line equivalent widths and to CN and CH band strengths. Although no significant differences among the CH, Ca, or Fe indices were found, the data do provide evidence of differences in CN band strengths. The CN bands show neither a bimodal distribution nor an anticorrelation with CH band strength, and thus are unlike the CN inhomogeneities seen in the globular clusters M~71 and 47~Tuc. We conclude that exploring the chemical evolution of the ISM using the CN band strengths of evolved stars from $\geq$10 Gyr old open clusters is questionable.
The genesis of the CN-inhomogeneity seen in NGC~6791 is unclear: examination of the unevolved stars of an old open cluster could help to resolve this issue. M 67 is another relatively old ($\sim$4.5 Gyr), populous open cluster, whose $1 M_{\sun}$ main sequence stars will in $\sim$5 Gyr evolve into a stage of evolution similar to that of the NGC~6791 RHB stars. We have obtained spectra of the 3600--4600 \AA\ wavelength region for twenty M 67 G dwarfs, and again measured CN, CH, Ca, and Fe line or band strength indices. No significant CN differences were found. This implies either that the differences seen among the 10-Gyr-old evolved stars of NGC~6791 resulted from stellar evolution processes effective during the most recent 5 Gyr of their evolution, or that the ISM 10 Gyr ago was more chemically inhomogeneous than it was 4.5 Gyr ago.
