Data points of the M31 sources included in the catalogue by G22 in the [3.6]−[4.5],[3.6]) plane obtained with the Spitzer filters are shown with grey points. Solid lines represent the same evolutionary tracks of some selected model stars reported in Fig. 1. The information regarding the C/O ratio, the DPR and the optical depth at 10 μm is shown in the left, central, and right panels, respectively.