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tokens: 2013 ), due to a likely very late thermal pulse (born-again scenario). Moreover, while most HE-PNe show N/O ratio larger than 1 (N rich), SuWt 2 has an N/O = 0.8 which makes it somewhat different. Most of the HE-PNe in our group (Tables 3 and 4 ) lie in the low-left corner of the PNe regime on the Hα/[N ii ] versus Hα/[S ii ] diagnostic diagram (Sabbadin, Minello Bianchini 1977 ; Gonçalves et al. 2003 ) of Galactic PNe (Fig. 9 ). Akras Gonçalves ( 2016 ) have recently demonstrated that in this regime also lie shock-excited regions, e.g. low-ionization structures such as knots and jets. Since these PNe do not exhibit high velocities, the scenario of shocks in such large and HE-PNe seems unlikely. Interestingly, some HE-PNe exhibit uncommonly bright [N i ] λ5200 and [O i ] λ6300 emission lines, which are strong indicators of shock activity. Recently, Ali et al. ( 2015 ) showed that a reverse shock of only 25 km s −1 in the PN G342.0–01.7, due to its motion through the ISM, can successfully explain the strong neutral emission lines. The low photoionization rate of their central stars, (very low L ) and large nebular size, would make the contribution of shocks easily perceptible even at small velocities (Akras Gonçalves 2016 ), or produce emission line spectra that resemble those of shock-excited regions (Raga et al. 2008 ). Figure 9. A diagnostic diagram of Galactic PNe by Sabbadin et al. ( 1977 ) with the density ellipses introduced by Riesgo López ( 2006 ) and for the HE-PNe in Tables 3 and 4 . Table 3. Chemical abundances † and dereddened emission lines of HE-PNe. † Chemical abundances are given in the notation of 12+log[ n (X)/ n (H)], †† These values have been derived from photoionization models. a Relative to Hβ= 100, b Average value from seven slit positions, c This value is a lower limit and corresponds only to N + . d This ratio was calculated based on the assumption of N/O = N + /O + , e The night sky emission line at 6300 Å may have not been properly subtracted. f They have been dereddened using R = 3.1 Table 4. Stellar and physical parameters of HE-PNe. † The stellar parameters have been derived from photoionization models. a These kinematical ages have been calculated by us, assuming a nominal expansion velocity of 40 km s −1 . b A possible born-again PN. 3 out of 11 HE-PNe are seen to exist outside the normal region in Fig. 9 , namely SBW 1, ESO 166–PN 21, and WeBo 1. SBW 1 and WeBo 1 are the youngest members in this group, and their central stars are such luminous that their high photoionization rate overcomes the shock excitation. Unfortunately, the stellar parameters of these HE-PNe are still unknown. Regarding ESO 166–PN 21, the [N ii ]/Hα and [S ii ]/Hα line ratios correspond to an average value from seven slit positions (for more details see in Peña et al. 1997 ). Studying the spectrum of each slit position separately, we deduce that the most distant regions (A1 and A5 in Peña et al. 1997 ) exhibit stronger [N ii ] and [S ii ]