**Table 5:** [O II] $\lambda 3729/\lambda 3726$ intensity ratios of low- $N_{\rm e}$ plasmas from literature.
Source	Obs. Pos	$I(\lambda3729)/I(\lambda3726)$	Ref.
IC 418	180 $^{\prime\prime}$	$\rm 1.79^{2.83}_{1.30}$	[1]
PX Pup	14 $^{\prime\prime}$	$\rm 1.43^{1.54}_{1.33}$	[1]
He 2-146		$\rm 1.43^{1.45}_{1.41}$	[2]
NGC 7293		$\rm 1.41^{1.43}_{1.39}$	[2]
IC 5148-50		$\rm 1.45^{1.49}_{1.41}$	[2]
NGC 6543	Halo	$\rm 1.44^{1.56}_{1.32}$	[3]
NGC 2371-2	9 $^{\prime\prime}$	$\rm 1.47^{1.73}_{1.21}$	[4]

References: [1] Monk et al. (1990), [2] Kingsburgh & English (1992),
[3] Middlemass et al. (1989), [4] Kingsburgh & Barlow (1992).

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