Optical observations of BL Lacertae in 2004-2005

We present optical observations of BL Lacertae (BL Lac) in the standard BVRI bands with the 80 cm Tsinghua Centre for Astrophysics-National Astronomical Observatories of China telescope. In 2004, BL Lac was monitored for 25 nights, between September and November, to study its long-term optical variability behaviour. The source initially faded by delta R similar to 1 mag in similar to 30 d and then brightened by delta R similar to 0.8 mag in similar to 20 d. It was further monitored for six consecutive nights on 2005 September 3-8, focusing on its intranight variability characteristics. The source continuously brightened by delta R similar to 0.7 mag during the six nights, with some visible fluctuations superimposed on to the brightness-increasing trend. The amplitudes of intranight variations tend to increase from I to B wavelengths. BL Lac exhibited strong bluer-when-brighter behaviour for the internight variations, which is less pronounced for some of the intranight variations. The intranight variations between different bands are correlated without measurable time lags in most time, but the flare on 2005 September 6 showed that the R-band variations lagged the B band ones by 1052(-759)(+859)s. Our results indicate that the optical variability properties of BL Lac (the prototype of low-energy-peaked BL Lac objects, LBLs) remarkably resemble the X-ray variability properties of high-energy-peaked BL Lac objects (HBLs). The similarities imply common origin of the variations, plausibly the most energetic tails of the synchrotron emission produced by the relativistic electrons in the jets, for both the optical emission of LBLs and the X-ray emission of HBLs.