Various abundance ratios as a function of O⁺⁺/(O⁺ + O⁺⁺) for PNe and H ii regions in NGC 300. PNe are represented by circles, compact H ii regions by stars and giant H ii regions by squares. The big red star is object no. 39, the big black circle is object no. 74 (see Sect. 4), and the big black star is object no. 57 (see Sect. 3.3). The objects represented by open circles are objects 12, 88 and 109 (see Sect. 3.4).