  VLBI observations have shown that the parsec-jet of 3C 345 is formed by
several components, ejected from the core with superluminal velocities and
travelling along bent trajectories on the plane of the sky. We interpret the
differences in velocity and position angle among the different features at
formation time as the result of parsec-scale precession of the relativistic jet
and calculate the aperture angle of the precession cone, the angle between the
cone axis and the line of sight and the Lorentz factor associated with the jet
bulk motion. We assumed a precession period of 10.1 yr, which is one of the
B-band light curve long-term periods reported in the literature. We propose
that boosting of the underlying jet emission, which is time-dependent due to
precession, is responsible for this long-term optical variability. Jet
precession with periods of several years can be produced in super-massive black
hole binary systems, when the secondary black hole is in an orbit non-coplanar
with the primary accretion disk, inducing torques in the inner parts of the
disk. Assuming that this mechanism is responsible for the jet precession in 3C
345, we estimate upper and lower limits for the masses of the two black holes,
as well as their mean separation. We found a correlation between the formation
of jet components and the occurrence of strong optical flares, as well as a
very strong anti-correlation between the intensity of these flares and the time
required for the components to reach the maximum flux density at radio
frequencies.
