SEDs of the four bright BL Lac object obtained from X-Shooter spectra in the wavelength range 3200−15 000 Å. The blue solid line is the sum of the two components: the non-thermal emission, which is described by a single power law (red dashed line), and the contribution from a luminous host galaxy starlight (M_r = −23.90) at z = 0.50 (green solid line). The superposition of the blue and red lines indicate that such a galaxy is hidden by the very bright nuclear emission.