Infrared and optical spectrum of a masquerading BL Lac from the sample, given in the observer’s frame. The gray points represent the broadband SED data (Paper IV). In red and purple, we show the result of the spectral decomposition following the optical spectroscopy campaign of the G20 sample (Paper III). In blue we show the template spectrum adapted from (Vanden Berk et al. 2001) and the respective error range, which comes from the uncertainty on the disk luminosity (cf. Fig. 2). At lower frequencies we see the infrared emission from a dust torus, and toward higher frequencies a thermal continuum from the accretion disk and broad lines from the BLR. Each of these three components is seen with a different relativistic boost in the rest frame of the relativistic jet, as explained in Section 2.1 and Appendix D.