The Whole Earth Blazar Telescope and the BL Lac Campaign 2000

We present R-band light curves of BL Lacertae from May 2000 to January 2001, obtained by 22 telescopes in 11 countries. More than 15000 UBVRI observations were performed in that period, which was the extension of the Whole Earth Blazar Telescope (WEBT) campaign originally planned for July-August 2000. The exceptional sampling reached allows one to follow the flux behaviour in fine details. Two different phases can be distinguished in the light curve: a first, relatively low-brightness phase is followed by an outburst phase, after a more than 1 mag brightening in a couple of weeks. Both the time duration (about 100 d) and the variation amplitude (roughly 0.9 mag) are similar in the two phases. Rapid flux oscillations are present all the time, involving variations up to a few tenths of mag on hour time scales, and witnessing an intense intraday activity of this source. In particular, a half-mag brightness decrease in about 7h was detected on August 8-9, 2000, immediately followed by an about 0.4 mag brightening in 1.7h.