2000MNRAS.319.1125G

Global {formmu1} polarization very long baseline interferometry (VLBI) observations of 18 BL Lacertae objects were obtained in 1992 March. The phased Very Large Array (VLA) was included among the antennas in the VLB array, and provided measurements of the integrated total flux and polarization of each of the sources during each 6.5-min VLBI scan. For nearly all the sources, these measurements were consistent with the fluxes and polarizations being constant during the 24-h VLBI experiment. However, in three objects (1334-127, 2131-021, 2155-152), the VLA measurements indicated that the integrated polarizations varied during the course of the VLBI observations. Subsequent analysis demonstrated that the variations in all three sources occurred in the milliarcsecond-scale compact radio structure. In the case of 2155-152, only one of two polarized milliarcsecond-scale features varied, while the polarization of the other feature was constant. These results suggest that the observed `intraday' polarization variability was dominated by variations intrinsic to the sources.