Equivalent widths in [Å] of the ¹³CO band head versus the ¹²CO band head. 47 Tuc stars are marked with asterisks. Black lines indicate effective temperature sequences of hydrostatic MARCS models for various constant log g values and three different values for the carbon isotopic ratio ¹²C/¹³C (as labelled). The temperature is decreasing in the direction of the small arrow. Coloured lines again show the location of the dynamical models with a carbon isotopic ratio of 7 assumed. In the case of L4CO25AP, we show also the results for differing ¹²C/¹³C ratios of 10 and 30 (light blue).