BL Lac Objects in a Complete Sample of Northern ROSAT All-Sky Survey Sources Mujica, R. ; Zickgraf, F. -J. ; Appenzeller, I. ; Krautter, J. ; Serrano, A. ; Voges, W. Abstract Publication: Highlights in X-ray Astronomy Pub Date: 1999 Bibcode: 1999hxra.conf..166M