Panel a: result from the spectrum synthesis fitting with SME. The observations are shown as dots, and we plot the best fits as blue and green lines for the ¹²CN and ¹³CN line region, respectively. This suggests an isotope ratio of 17.8 ± 1.9. The synthetic spectra for ±3σ deviations are shown as orange and red lines, respectively. Panel b: spectrum synthesis with MOOG for consistency of the spectral fitting by SME. See text for the parameters of the underlying MARCS model. Panel c: observations minus synthesis (O-S) for the MOOG synthesis. Error bars corresponds to an S/N of 1000.