negligible, and since the ionization potentials of O$^{++}$ and He${^+}$ are very similar (54.93 eV and 54.42 eV, respectively), we do not expect an important amount of O$^{3+}$ in these nebulae. Therefore, in these low-ionization PNe, we simply add the ionic abundances of O$^{+}$ and O$^{++}$ to calculate the oxygen abundance. For the other PNe we use the expression

$$\label{eq:ICF_O}
\frac{\mbox{O}}{\mbox{H}} = \left( \frac{\mbox{O}^{+} + \mbox{O}^{++}}{\mbox{H}^{+}}\right)\left(\frac{\mbox{He}^{+} + \mbox{He}^{++}}{\mbox{He}^{+}}\right)$$

[@Peimbert_71]. The ICFs of oxygen in most of our sample PNe have values below 1.2. The exceptions are the PNe with the highest $I(\mbox{\ion{He}{2}~}\lambda4686)/I(\mbox{H}\beta)$ ratios: IC 2165 (ICF = $1.98$), NGC 3242 (1.28), NGC 3918 (1.51), NGC 6741 (1.40), NGC 6818 (1.97), and NGC 7662 (1.57). The differences between the values of O/H derived from equation (\[eq:ICF\_O\]) and those derived with the ICF suggested in @DelgadoInglada_14 are lower than 0.02 dex for most of the nebulae, and reach a maximum value of 0.13 dex for IC 2165 and NGC 6818. The differences are small because both ICFs lead to similar corrections for low ionization PNe, with $\mbox{He}^{++}/(\mbox{He}^{+}+\mbox{He}^{++})<0.5$.

In general, our values of O/H agree within 0.1 dex with those derived in the papers listed in Table \[tab:1\]. Ten PNe show differences of 