C/O abundance ratio plotted against C abundance for our sample of PNe. The horizontal dashed line represents the separation between C-rich and O-rich objects. The predictions of stellar evolution models by Karakas (2010) (red models: Z = 0.02, magenta models: Z = 0.008) and by Marigo (2001) (green models: Z = 0.02) for stars with different initial masses (indicated in units of M⊙) are included.
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