Metallicity-luminosity relation for external HII regions in Stephan’s Quintet (yellow open circles; Mendes de Oliveira et al. 2004), nearby irregular galaxies (black plusses, Pilyugin et al. 2004; black asterisks, van Zee & Haynes 2006), TDG candidates (green filled triangles, Weilbacher et al. 2000, 2003; red crosses, Duc & Mirabel 1998; blue open squares, Temporin et al. 2003) and our complexes (black dots with errors). The line shows the correlation found by van Zee. Only the luminosities given by Pilyugin are corrected from internal extinction. For those Hα complexes for which an estimate of the extinction has been made (see Sect. 5.2.1), the metallicity-luminosity relation is also shown (dots with dashed line errors).