  Context. Capella ({\alpha} Aur) is one of the few binaries in the sky with
two cool giant stars. With spectral types of G8III and G0III, the two
components appear at different but distinct stages in their evolution. The G0
secondary star is a Hertzsprung-gap giant, and the G8 primary star is thought
to be a clump giant. Aims. We present a new measure of the carbon 12 C/ 13 C
isotope ratio of the primary component of Capella using high-resolution R
$\approx$250 000 spectra obtained with the Potsdam Echelle Polarimetric and
Spectroscopic Instrument (PEPSI) with both the Vatican Advanced Technology
Telescope (VATT) and the Large Binocular Telescope (LBT). Methods.
Signal-to-noise ratios of up to 2 700 were obtained by averaging nightly
spectra. These average spectra were used to disentangle the two binary
components. The isotope ratio was derived with the use of spectrum synthesis
from the CN lines at 8004 {\AA}. Results. We found that the 12 C/ 13 C ratio of
the primary component of Capella is 17.8$\pm$1.9. Our measurement precision is
now primarily limited by the spectral-line data and by the grid-step size of
the model atmospheres rather than the data. The separated spectrum of the
secondary component does not show distinguishable 12 CN and 13 CN lines because
of its v sin i and higher temperature. Conclusions. Our new 12 C/ 13 C value is
significantly lower than the previous value of 27$\pm$4 but now agrees better
with the recent model prediction of 18.8 - 20.7.
