Top: sketch of the stellar system LS I + 61°303. We assume two ejections of relativistic electrons along the orbit as in Bosch-Ramon et al. (2006). The two ejections refill the conical precessing jet defined in Massi & Torricelli-Ciamponi (2014). The electrons emit by synchrotron and IC processes. The resulting jet parameters are given in Table 2. The ejection at periastron gives rise to gamma-ray emission only and the periodogram of this emission only shows P₁, the orbital period. The ejection towards apastron is associated with gamma-ray and radio emission. The periodograms for both emissions show two features, at P₁ and P₂, the precession period of the jet. a) Lomb-Scargle periodogram for Fermi-LAT data from the orbital phase interval Φ = 0.0−0.5 (periastron). The orbital period P₁ is clearly detected. b) Lomb-Scargle periodogram of the model gamma-ray data (IC) from the orbital phase interval Φ = 0.0−0.5 (periastron). The orbital period P₁ is the only peak, in agreement with the observations. c) In the orbital phase interval Φ = 0.5−1.0 (apastron), there is a peak at the precession period P₂ in addition to the orbital period P₁. d) The two peaks at P₁ and P₂ in the apastron data are reproduced by the model. e) Lomb-Scargle periodogram for OVRO radio data for the whole orbital phase interval. Both P₁ and P₂ are present. f) Lomb-Scargle periodogram of the model radio data (synchrotron emission). Both peaks are reproduced by the model.