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Context. Capella ({\alpha} Aur) is one of the few binaries in the sky withtwo cool giant stars. With spectral types of G8III and G0III, the twocomponents appear at different but distinct stages in their evolution. The G0secondary star is a Hertzsprung-gap giant, and the G8 primary star is thoughtto be a clump giant. Aims. We present a new measure of the carbon 12 C/ 13 Cisotope ratio of the primary component of Capella using high-resolution R$\approx$250 000 spectra obtained with the Potsdam Echelle Polarimetric andSpectroscopic Instrument (PEPSI) with both the Vatican Advanced TechnologyTelescope (VATT) and the Large Binocular Telescope (LBT). Methods.Signal-to-noise ratios of up to 2 700 were obtained by averaging nightlyspectra. These average spectra were used to disentangle the two binarycomponents. The isotope ratio was derived with the use of spectrum synthesisfrom the CN lines at 8004 {\AA}. Results. We found that the 12 C/ 13 C ratio ofthe primary component of Capella is 17.8$\pm$1.9. Our measurement precision isnow primarily limited by the spectral-line data and by the grid-step size ofthe model atmospheres rather than the data. The separated spectrum of thesecondary component does not show distinguishable 12 CN and 13 CN lines becauseof its v sin i and higher temperature. Conclusions. Our new 12 C/ 13 C value issignificantly lower than the previous value of 27$\pm$4 but now agrees betterwith the recent model prediction of 18.8 - 20.7.

Carbon 12C/13C isotope ratio of alpha Aurigae revised