Spectra of the optical counterparts of four BL Lacs in our sample. For each panel, the upper box reports the observed spectrum not corrected for the intervening Galactic absorption, while the lower one shows the same spectrum normalized to its continuum. The main spectral features are labeled in the lower panels, with the intervening Galactic features indicated in the lower part of the box and (if present) the features allowing the redshift measurement marked in the above part of it. The symbol ⊕ indicates atmospheric telluric absorption bands. The TNG spectra have been smoothed using a Gaussian filter with σ = 3 Å.
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