7 Conclusions

The results of multicolour observations of OJ 287 carried out in the frame of OJ-94 Project were analysed by Choloniewski's method with the aim to study the properties of variable sources responsible for photometric behaviour of the object.

It is found that in the optical region the sources have the same power-law spectrum $(F_\nu\sim\nu^{-1.5})$ irrespective of the flux level (both in and out of flare). This spectrum is of synchrotron nature. In IR the spectrum flattens during the flare possibly because of synchrotron self-absorption.

Comparison of the outburst of 1994 with those of 1971 and 1983 shows that there is a clear correlation between power of outburst and spectral energy distribution of variable source in each event; the larger is the outburst, the bluer is the variable source, i.e. in more powerful outbursts the energy spectrum of relativistic electrons is flatter.

Acknowledgements

The authors are grateful to all participants of OJ-94 Project for collaboration. V.A.H.-Th. would like to thank for hospitality the Tuorla Observatory where this work was partly completed. LOT, AS and TP would like to thank Finnish Academy for support, V.A.H.-Th. and SGM are grateful for financial support to Russian Federal Program "Astronomy" and Russian Fund of the Basic Researches (grant 98-02-16609).