  OJ287 is a BL Lac object at redshift z=0.306 that has shown double-peaked
bursts at regular intervals of ~12 yr during the last ~ 40 yr. Due to this
behavior, it has been suggested that OJ287 might host a close supermassive
binary black hole. We present optical photopolarimetric monitoring data from
2005-2009, during which the latest double-peaked outburst occurred. We find a
stable component in the optical jet: the optical polarization core. The optical
polarization indicates that the magnetic field is oriented parallel to the jet.
Using historical optical polarization data, we trace the evolution of the
optical polarization core and find that it has showed a swing in the Stokes
plane indicating a reorientation of the jet magnetic field. We also find that
changes in the optical jet magnetic field seem tightly related to the
double-peaked bursts. We use our findings as a new constraint on possible
binary black hole models. Combining all available observations, we find that
none of the proposed binary black bole models is able to fully explain the
observations. We suggest a new approach to understanding OJ287 that is based on
the assumption that changes in the jet magnetic field drive the regular
outbursts.
