  Optical circular polarization observations can directly test the particle
composition in black holes jets. Here we report on the first observations of
the BL Lac type object S4 0954+65 in high linear polarized states. While no
circular polarization was detected, we were able to place upper limits of <0.5%
at the 99.7% confidence. Using a simple model and our novel optical circular
polarization observations we can constrain the allowed parameter space for the
magnetic field strength and composition of the emitting particles. Our results
favor models that require magnetic field strengths of only a few Gauss and
models where the jet composition is dominated by electron-positron pairs. We
discuss our findings in the context of typical magnetic field strength
requirements for blazar emission models.
