Composite spectrum of the 74 spectra of the SFG sample (see Fig. 5): zoomed image of the three detectable stellar photospheric absorption lines (C III λ1176 Å, O IV λ1343 Å, S V λ1501 Å. The latter was not used in the analysis since it is too weak to be reliable measured), the C III] λλ1906,1908 Å nebular emission line doublet and the CII] λ2326 Å nebular line. The vertical lines indicate the vacuum wavelength of the line.