Time Variable Faraday Rotation Measures of 3c 273 & 3c 279

Multifrequency polarimetry with the VLBA confirms the previously reported timevarying 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 year time-scales. Variation in the RM properties of quasars may result from a Faraday screen which changes on time-scales of a few years, or from the motion of jet components which 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. Subject headings: galaxies: active – galaxies: ISM – galaxies: jets – galaxies: nuclei – quasars: general – radio continuum: galaxies