Observed (dotted red line) and synthetic (solid blue line) spectra in the region around the ¹²CN and ¹³CN lines at ~8004 Å  for η Her. Synthetic spectra were calculated for four ¹²C/¹³C ratios, 10, 16, 22 and 32. Bottom part: observed spectrum of η Her and spectrum of a hot star (cyan line) used to to “dry" the observed spectrum. Upper part: observed spectrum without the contribution from the telluric H2O lines.