SIMBAD references

We present the results of UBVRI photometry of stars in the fields of 13 BL Lacertae objects and quasars. The calibrated stars are intended to facilitate photoelectric and CCD differential photometry of these highly variable active galactic nuclei (AGNs). The magnitudes of most of the stars were measured on more than two occasions, and we provide two to five stars per field. Several stars are common to those calibrated by previous observers, although the earlier efforts do not cover all five bandpasses. Our results for these stars are typically in close agreement with previous measurements obtained using common filters. In addition, we present UBVRI magnitudes for stars in the fields of four AGNs (CTA 26, 0716+332, OV-236, and PKS 2345-167) with no previously published comparison stars.