Comparison between chemical kinetics model predictions (Sect. 6) and the CO radial abundance distributions from our line radiative transfer analysis (Sect. 5.1) for the two X-AGBs: TDra (top) and EY Hya (bottom). The black solid line represents ‘model-uv/x’, the green dotted line represents ‘model-x’, the grey dashed line represents ‘model-std’, the orange dash-dotted line represents ‘model-uv’. The blue solid line shows the derived CO radial abundance distribution from the radiative transfer modelling of the observed CO lines.