SIMBAD references

Stirling et al. recently reported the discovery of a 2.3 yr periodic variation in the structural position angle of the parsec-scale radio core in the blazar BL Lac. We searched for independent confirmation of this periodic behavior using 43 GHz images of the radio core during 10 epochs overlapping those of Stirling et al. Our maps are consistent with several periodicities, including one near the period reported by Stirling et al. By comparing our position angle measurements with those of Stirling et al., we find strong, consistent evidence for position angle variations of the inner core during the observed epochs. However, the claim of periodic variation is not convincing, especially when the most recent epochs (2000.60-2003.78) are included. A definitive resolution will require continued monitoring of the core structure over several periods.