Comparison between the metallicity 12 +log (O/H) and log(U), both derived with the code HCm. The plot shows the anti-correlation between the two quantities, with the limited grid of models (green continuous line) considered by HCm when only [O iii]3727 Å and [O iii]5007 Å are available. Red points are the galaxies in our sample [O iii]4363 detection, showing that the empirical relation is real, since HCm does not introduce any assumptions in this case.