12C/13C in atmospheres of red giants and peculiar stars

We determine the carbon isotopic ratios in the atmospheres of some evolved stars in both globular clusters and the disk of our Galaxy. Analysis of 12 CO and 13 CO bands at 2.3 micron was carried out using fits to observed spectra of red giants and Sakurai's object (V4334 Sgr). The dependence of theoretical spectra on the various input parameters was studied in detail. The computation of model atmospheres and a detailed abundance analysis was performed in a self-consistent fashion. A special procedure for determining the best fits to observed spectra was used. We show, that globular cluster giants with [Fe/H] $\,{ 1.3 have a low 12 C/ 13 C = 4 ±1 abundance ratios. In the spectra of Sakurai's object (V4334 Sgr) taken between 1997-98, the 2.3 micron spectral region is veiled by hot dust emission. By fitting UKIRT spectra we determined 12 C/ 13 C = 4 ±1 for the July, 1998 spectrum. CO bands in the spectra of ultracool dwarfs are modelled as well.