BL Lac Objects and Relativistic Beaming Model
  In this paper: 1. The assumption of the dependence of Doppler factor on the emission frequency (Fan et al. 1993) is used to explain the observational differences between the radio-selected BL Lac objects(RBLs) and the X-Ray-Selected BL Lac objects (XBLs): a) Hubble relation; b) different multiwavelength correlations; c) different regions in the effective spectral index diagram; d) different polarization. The results suggest that RBLs and XBLs are the same.   2. From the analysis of the relation between infrared magnitude and redshift, it is proposed that the parent population of BL Lac objects should be FRI radio galaxies and FRII(G) radio galaxies showing the optical spectra of a galaxy.   3. From the superluminal motion, the assumption (Fan et al. 1993) is confirmed.   4. Based on the relation between polarization and Doppler factor (Fan et al. 1997, A&A, 327, 947), it is proposed that the ratio of the beamed luminosity to the unbeamed luminosity in the source frame of OVVs/HPQs is smaller than that of BL Lac objects. 
J.H. Fan