Left: relation between the Dn(4000) and Hδ indices from the SDSS subsample of 21 065 galaxies. For each bin in Dn(4000), the black dots indicate the mean value and dispersion of the best Gaussian fit to the Hδ distribution. Right: Hδ versus Dn(4000) for the outliers of the Dn(4000) versus W2−W3 relation (black dots). The red dots label, for each bin in Dn(4000), the mean value and dispersion of the best Gaussian fit to the Hδ distribution for the overall SDSS galaxies population. The straight blue line is the expected locus of BL Lacs, obtained by varying the ratio between jet and galactic emission (see text for details).