Lomb-Scargle periodograms of three independent long-term monitorings of LS I +61°303. Top: OVRO at 15 GHz. Centre: Fermi-LAT in the energy range 100 MeV to 300 GeV using only data from the orbital phase interval Φ = 0.5−1.0 (see Jaron & Massi, 2014). Bottom: GBI at 2 GHz (empty circles) and 8 GHz (filled squares) (see Massi & Jaron, 2013). In all periodograms there are two periodicities with a false-alarm probability of between 0.00 and 0.01 (see Sect. 2). The ratio (R) between the intensity of the two spetral features at P₁ and P₂ is different in the different periodograms and is R = 1.9 at 2 GHz and R = 1.5 at 8 GHz for GBI data, R = 1.4 at 15 GHz and R = 1.3 for GeV data.