Time-variable Faraday rotation measures of 3C 273 and 3C 279

Multifrequency polarimetry with the Very Long Baseline Array confirms the previously reported time-varying Faraday rotation measure (RM) in the quasar 3C 279. Variability in the RM and electric vector position angle (EVPA) of the jet component (C4) is seen, making it an unreliable absolute EVPA calibrator. 3C 273 is also shown to vary its RM structure on 1.5 yr timescales. Variation in the RM properties of quasars may result from a Faraday screen that changes on timescales of a few years or from the motion of jet components that sample spatial variations in the screen. A new component emerging from the core of 3C 279 appears to be starting to sample such a spatial variation. Future monitoring of this component and its RM properties is suggested as a diagnostic of the narrow-line region in 3C 279. We also present a new method of EVPA calibration using the VLA Monitoring Program.