6 Conclusions

We have reported the results of a systematic $BVR_{\rm c}I_{\rm c}$ monitoring of the BL Lac object ON 231 in the period March 1994 - March 1997. The source brightness was at the highest level ever observed. The flux behaviour was characterized by three major bursts, separated each other by about 300 days, in which ON 231 reached practically the same maximum level (historical maximum), and with a total amplitude of about 1.5 mag in all the four bands. Superimposed to each burst there is a series of flares with a typical duration of 20-60 days. Variations of a few tenths of magnitude lasting about a week are always present in the light curve. More rapid events with time scales of 2-3 days or less were mainly observed when the source was in a faint state.

We have found a positive indication of a correlation between the spectral index and the source optical luminosity, in the sense that the spectrum becomes flatter when the flux increases, which could be related to the aging of the synchrotron emitting electrons.

Acknowledgements

The Perugia group is deeply indebted to P.Maffei without whose support this work would have not been possible. This research has made use of the Simbad database, operated at CDS, Strasbourg, France and of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.