5 Summary

We have presented interferometric radio data at 1.36, 1.66 and 4.8 GHz with the VLA at A, B and D configurations, and at 1.4 GHz with the WSRT, on 28 of 34 objects of the 1 Jy sample of BL Lac objects (Stickel et al. 1991). We have obtained high sensitivity radio images at the arcsec resolution, in order to evaluate the extended luminosity of these objects, most of which had poor or no radio data at this resolution. We found that most of the sources observed possess substantial extended emission, and often our flux densities exceed those previously reported into the literature. A few sources have unbeamed radio luminosities at 1.36 GHz of the order of 10²⁷ W Hz^-1, supporting the hypothesis that the parent population of BL Lac objects is a mixture of FR I and FR II radio sources.

Acknowledgements

DD and MB acknowledge financial support from the European Union as an EU Fellow under contract CHBGCT920212. This work was partly supported by the Italian Ministry for University and Research (MURST) under grant Cofin98-02-32 The National Radio Astronomy Observatory is operated by Associated University Inc under contract with the National Science Foundation. The WSRT is operated by the Netherlands Foundation for Research in Astronomy (NFRA) with financial support by the Netherlands Organization for Scientific Research (NWO). We thank the referee, Travis Rector, whose comments significantly improved the paper.