4 Conclusions

In this paper we have presented photometric sequences in the fields of 10 BL Lacs that are part of the object list of the Torino monitoring group. Standard magnitudes have been determined in BVR for 45 stars and in VR for 11 ones. All stars are brighter than V=16 and can be used by observers to derive the BL Lac magnitudes.

Acknowledgements

We are grateful to G. De Francesco for her collaboration during observations and data reduction. We wish to thank F. Scaltriti and G. Massone for useful discussions. This research has made use of the STScI Digitized Sky Survey (DSS) 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. We thank the referee (Dr. Véron) for his prompt report and useful comments.