Ratio of ¹⁷O/¹⁸O (dashed black line) for a model star of 1.5 M_⊙ and metallicity Z = 0.02 as a function of time since the first thermal pulse in the TP-AGB phase as obtained with the STARS stellar evolution code presented in Eggleton (1971), Stancliffe et al. (2004), Stancliffe & Eldridge (2009). The helium burning luminosity is overplotted (solid gray line) to indicate the various thermal pulses. This model does not include any extra mixing or convective overshooting. The ¹⁷O/¹⁸O ratio varies insignificantly over the entire TP-AGB phase.