Spectral synthesis fits (dotted curves) of the C₂ features around 4740 Å obtained with the adopted C abundance and ¹²C/¹³C value (dotted curve). The observed spectrum is shown by a solid curve. Two alternative fits with ¹²C/¹³C ∼ 1 (short-dashed line) and 12 (long-dashed line) are shown to illustrate the sensitivity of the line strengths to the isotopic carbon abundance ratios.