  Although the continua of radio-loud Active Galactic Nuclei (AGN) are
typically dominated by synchrotron radiation over virtually the entire
spectrum, it is not clear whether the radio and higher-frequency emission
originate in the same or different parts of the jet. Several different
radio--optical correlations based on polarization data have been found
recently, suggesting that the optical and radio polarization may be closely
related, and that the corresponding emission regions may be cospatial (Gabuzda
et. al2006, Jorstad et al. 2007, D'Arcangelo et al. 2007) Our joint analysis of
optical and VLBA polarization data for a sample of about 40 AGNs shows that,
after correction for the inferred VLBA core Faraday rotations, most BL Lac
objects and some quasars have aligned VLBA-core and optical polarizations,
although many quasars also show no obvious relationship between their VLBA-core
and optical polarization angles. This may indicate that not all AGNs have
cospatial regions of optical and radio emission in their jets. However, another
possibility is that some of the 7mm-2cm VLBA cores have Faraday rotations of
the order of several tens of thousand of rad/m^2, which were not properly fit
by our three-frequency data due to n*pi ambiguities in the observed
polarization angles, leading to incorrect subtraction of the effects of the
core Faraday rotation, and so incorrect "zero-wavelength" radio polarization
angles. The possibility of such high core Faraday rotations is supported by the
results of the parsec-scale Faraday-rotation studies of Zavala & Taylor (2004)
and Jorstad et al. (2007).
