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Periodicities in blazar light curves may be related to helical trajectoriesin extragalactic radio jets by differential Doppler boosting effects. Weconsider ballistic and non-ballistic (i.e., radial) trajectories and discussthree 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 ballisticmotion is unlikely to result in observable periods of less than several tens ofyears. We demonstrate that for non-ballistic helical motion the observed periodis generally strongly shortened relative to the real physical driving periodbecause of light-travel time effects. Internal jet rotation may thus accountfor observed periods $P_{\rm obs} \leq 10$ days. Periodicity due toorbital-driven (non-ballistic) helical motion, on the other hand, is usuallyconstrained to periods of $P_{\rm obs} \geq 10$ days, while Newtonian-drivenprecession is unlikely to be responsible for periodicity on a timescale $P_{\rmobs} \leq 100$ days but may well be associated with periods of $P_{\rm obs}\geq 1$ yr.Comment: 10 pages, ApJ Letters in pres

On the Geometrical Origin of Periodicity in Blazar-type Sources