Multi-Wavelength Observations on the Gamma-Ray Blazar PG1553+113 as a Probe for Geometrical Periodical Modulation

New claims of periodic variability from gamma-ray blazars have been reported, possibly pointing at milli-pc SMBH binary systems. A modulation of ~2 year on 3.5 cycles was recently discovered with Fermi/LAT on the blazar PG1553+113 -- for the first time in gamma-rays with high significance -- and confirmed by optical lightcurves. Other possible gamma-ray periodic variations have been claimed on PKS 2155-304 and PKS 0537 by Sandrinelli et al. (2015, 2016). The interpretation of such periodicity, when confirmed with continuous observations in following years, is not straightforward. Emission from blazars is dominated by non-thermal emission from the jet; different processes in the jet or at its base, may lead to quasi-periodic emission. We used multi-wavelength (MWL) observations on PG1553+113 to investigate if the observed modulation can be explained with geometrical variations in the jet, possibly pointing to jet precession or to an helical pattern. The ongoing MWL monitoring campaign from radio to very-high energy gamma-ray bands, led by the MAGIC collaboration, will follow the maximum expected at the beginning of 2017, and will allow to set tighter constrains on underlying periodic processes