Photometric monitoring of three BL Lacertae objects in 1993-1998 J. M. Bai, G. Z. Xie, K. H. Li, X. Zhang and W. W. Liu Astron. Astrophys. Suppl. Ser., 136 3 (1999) 455-460 Published online: 15 May 1999 DOI: 10.1051/aas:1999227