Evolutionary tracks calculated with the stellar evolution code ATON (Ventura et al. 1998) for different masses (different colors), expressed at the beginning of the AGB phase. The solid tracks are the Z = 4 × 10⁻³ (left panel) and Z = 2 × 10⁻³ (right panel) and the dashed lines the Z = 8 × 10⁻³ (left panel) and Z = 1 × 10⁻³ (right panel). The gray circles are the PN sample, with the luminosities and the effective temperatures derived in the present paper. Full and open symbols refer to C/O ratios greater and lower than unity, respectively. Due to the large oxygen error bar, the C/O of SMP LMC 34 is undefined.