Optical and UV observations of the BL Lacertae object PKS 2155-304.

The BL Lac object PKS 2155-304 has been monitored in two subsequent observing campaigns within three weeks in June 1990 and during another run in September 1994 at optical wavelengths. During the first part PKS 2155-304 showed night-to-night variations of a few percent, whereas in the second part no variability could be detected. During the third run PKS 2155-304 showed variations similar to those in the first campaign. We did not detect any variations on time-scales shorter than one day. The object was in a bright state during the observations in June 1990 (m_V_~12.7) and close to its historical maximum brightness in September 1994 (m_V_~12.3). The observations imply that even objects with a high duty cycle and a well known history of rapid variability may be quiescent over longer periods. The power density spectra are not constant at high frequencies (time-scales shorter than a few days) and are not related to the average brightness level at a particular epoch. Comparison with IUE spectra in June 1990 show that PKS 2155-304 was also very bright in the UV. During that period we found an indication that the optical-UV spectral index steepens, when the source became brighter, whereas usually the opposite is observed.