  Periodicities in blazar light curves may be related to helical trajectories
in extragalactic radio jets by differential Doppler boosting effects. We
consider ballistic and non-ballistic (i.e., radial) trajectories and discuss
three possible periodic driving mechanisms for the origin of helical jet paths,
namely, orbital motion in a binary black hole system (BBHS), jet precession,
and intrinsic jet rotation. It is shown that precessional-driven ballistic
motion is unlikely to result in observable periods of less than several tens of
years. We demonstrate that for non-ballistic helical motion the observed period
is generally strongly shortened relative to the real physical driving period
because of light-travel time effects. Internal jet rotation may thus account
for observed periods $P_{\rm obs} \leq 10$ days. Periodicity due to
orbital-driven (non-ballistic) helical motion, on the other hand, is usually
constrained to periods of $P_{\rm obs} \geq 10$ days, while Newtonian-driven
precession is unlikely to be responsible for periodicity on a timescale $P_{\rm
obs} \leq 100$ days but may well be associated with periods of $P_{\rm obs}
\geq 1$ yr.
