3 Conclusions

In this short paper we have reported optical data for five X-ray selected BL Lacertae objects. The data, taken with the $1.05\rm\,m$ telescope of the Torino Astronomical Observatory from February 29, 1996 to January 23, 1999, are in the Johnson's B, V, and Cousins' R bands. In only one case (1ES 1959+650) variability on a time scale of a few days was observed.

The data presented in this paper represent an important contribution to the study of these objects, as optical observation of them was seldom performed in the past. All of our observation periods include pointings of the BeppoSAX satellite, and in some cases our observations are strictly simultaneous with them. This will allow an easy comparison of our optical data with the X-ray ones and also with those at other wavelengths, if available. In this way, information on the emission processes of the BL Lacs jets can be derived.

Our data confirm that generally the X-ray selected BL Lacs do not present strong optical variability, in comparison with the radio selected ones.

Acknowledgements

This work was partly supported by the Italian Ministry for University and Research (MURST) under grant Cofin98-02-32 and by the Italian Space Agency (ASI).