Constraints on magnetic field and particle content in blazar jets through optical circular polarization

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