Linear polarization and composition of VLBI jets
  It is shown that linear polarization data can be used to constrain the composition (normal or pair plasma) of pc-scale extragalactic jets.   A simple criterion, based on synchrotron and Faraday depolarization properties, is established. It does not depend on the particle density and the length of the emitting region along the line of sight, thus eliminating two physical unknowns. 
Didier Fraix-Burnet