Comparison of the abundances of carbon and oxygen observed for the PN sample (gray circles) and those computed using the ATON code at the end of the AGB evolution (colored symbols). Different shapes of the model represent different metallicities. Full and open symbols refer to the C/O ratio greater and lower than unity, respectively. The C/O of SMP LMC 34 is undefined due to the large oxygen error bar. The dashed line separates the oxygen-rich sources from the carbon-rich ones.