Evolution of radius (black lines, scale on the left) and surface C/O ratio (red, scale on the right) during the AGB phase of stars of initial masses 1.1 M_⊙ (left panel), 2.5 M_⊙ (middle), and 3 M_⊙ (right). The thin, blue horizontal lines indicate the C=O condition, whereas the green arrows indicate the phase of rapid expansion of the star, as a consequence of the surface carbon enrichment.