Equivalent widths in [Å] of the OH line at 15 573 Å versus the strength of the ¹²CO band head. Black lines mark temperature sequences for log g = 0.0 and two different C/O ratios. The two coloured lines show the behaviour of the dynamic models L4C048AP and L4C025AP. Maximum and minimum phases are indicated (Φ_bol = 0.0 and 0.5, respectively). For all calculations a ¹²C/¹³C ratio of 10 was assumed.