Equivalent widths in [Å] of the OH line at 15573 Å versus the strength of the ¹²CO band head. The 47 Tuc stars (Table 1) are marked with asterisks. Sequences of hydrostatic models (with varying T_eff but constant log g) are shown as black lines, the corresponding log g values are labelled at the top of these lines. The small arrow indicates the trend of decreasing temperature. Coloured lines mark the location of selected dynamical models during the light cycles. For all models, the carbon isotopic ratio ¹²C/¹³C was set to 7. The C/O ratio is 0.25 if not stated otherwise.