  Optical variability of 3C 120 is discussed in the framework of jet
precession. Specifically, we assume that the observed long-term periodic
variability is produced by the emission from an underlying jet with a
time-dependent boosting factor driven by precession. The differences in the
apparent velocities of the different superluminal components in the
milliarcsecond jet can also be explained by the precession model as being
related to changes in the viewing angle. The evolution of the jet components
has been used to determine the parameters of the precession model, which also
reproduce the helical structure seen at large scales. Among the possible
mechanisms that could produce jet precession, we consider that 3C 120 harbours
a super-massive black hole binary system in its nuclear region and that torques
induced by misalignment between the accretion disc and the orbital plane of the
secondary black hole are responsible for this precession; we estimated upper
and lower limits for the black holes masses and their mean separation.
