  Polarization offers a unique view in the physical processes of astrophysical
jets. We report on optical circular polarization observations of two famous
blazars, namely 3C 279 and PKS 1510-089, at high linearly polarized states.
This is the first time PKS 1510-089 is observed in optical circular
polarization. While only upper limits can be extracted from our observing
campaign, the non-detection of optical circular polarization allows us to
provide meaningful constraints on their magnetic field strength and jet
composition. We find that high-energy emission models requiring high magnetic
field strength and a low positron fraction can be excluded.
