Left: Time variation of the luminosity (black line, scale on the left) and the J magnitude (green, scale on the right) of a model star of initial mass 2.5 M_⊙ and metallicity Z = 0.008. The black arrow indicates the start of the C-star phase. Right: Time evolution of the surface C/O and the DPR of the same model star shown in the left panel. Times are counted since the beginning of the TP-AGB phase. Grey-shaded regions indicate the phase during which 1.5 <  J − K <  2 mag.