C abundance and ¹²C/¹³C from the CH G-band in the UVES 390 spectrum. Left panel: region around the two features that are dominated by ¹³CH, one of which is used to pinpoint ¹²C/¹³C (blue rectangle). The bluer feature at ∼4230 Å was not considered due to an unidentified blend (see main text). The observed spectrum is represented by black dots connected by gray lines and the best-fit synthesis (red) and its abundance error margin of 0.05 dex are depicted in blue, respectively. The dashed spectrum shows a synthesis without any C. Right panel: same as left panel but in the range used to constrain the C (CH) abundance.