A study of six low redshift QSO pairs
  The dynamical properties of six SDSS quasar pairs at z < 0.8 are investigated. The pairs have proper transverse separation < 500 kpc, and velocity difference along the line of sight < 500 km/s. If they are bound systems their dynamical mass can be evaluated and compared with that of host galaxies. Evidence is found of an excess of the former mass with respect to the latter. This suggests that these quasar pairs are hosted by galaxies with massive dark halos or that they reside in a group/cluster of galaxies. 
E. P. Farina, R. Falomo and A. Treves
astro-ph.CO