Expected fraction of BL Lac candidates recovered with our selection method at various levels of radio luminosity obtained with Monte Carlo simulations. The various lines refer to different assumptions on the radio–optical spectral index. Red: α_ro = 0.49, the mean obtained from the 35 BL Lac candidates with 0.05 <z ≤ 0.15; green: α_ro = 0.53, the mean of the candidates with log L_r> 40.5erg s^-1; blue: α_ro = 0.59, the mean of the 1Jy BL Lacs.