the He$^{+}$ and He$^{++}$ abundances. Hence, we derive all these ionic abundances using the densities and temperatures from Table \[tab:1\], in particular [$T_{\rm e}$]{}[\[\]]{} for O$^{+}$, Fe$^{+}$, and Fe$^{++}$, and [$T_{\rm e}$]{}[\[\]]{} for O$^{++}$, He$^{+}$, and He$^{++}$.

The values of O$^{+}$/H$^{+}$ and O$^{++}$/H$^{+}$ are calculated using the intensities of the \[\] $\lambda\lambda3726+29$ and \[\] $\lambda\lambda4959,5007$ lines with respect to H$\beta$ and the routine [*ionic*]{} of the package [*nebular*]{} in [IRAF]{} with the changes in atomic data mentioned in § \[sec:phys\_cond\].

The values of He$^{+}$/H$^{+}$ are based on $\lambda$6678 and the calculations of @Benjamin_99. In DdDm 1 and IC 2165 we replace our computed He$^{+}$ abundances (He$^{+}$/H$^{+}=0.041$ and 0.042) with the average of the He$^{+}$ abundances derived from the $\lambda$$\lambda$4471, 5876 lines by @Hyung_94 and @Wesson_05, respectively. These authors suggest that the low values implied by the $\lambda$6678 line could be due to underlying absorption.

The values of He$^{++}$/H$^{+}$ are derived using $\lambda$4686 and the emissivities of @Storey_95. The emissivities are also from @Storey_95.

We estimate the Fe$^{+}$ abundances for eleven PNe using the emissivities of @Bautista_96 and \[\] $\lambda8616$, one of the few \[\] lines which are not affected by fluorescence [@Verner_00]. In some cases where this line is not 