Long-term modulation in LS I +61°303. 8.3 GHz GBI observations (red/squares) and model (black/stars) data. a) Model and data vs. time (bottom x-axis) and vs. the long-term modulation phase Θ (top x-axis). The long-term modulation phase Θ is the fractional part of (t − t₀)/1667 with t₀ = JD 2 443 366.775 (Gregory 2002). The GBI data are averaged over 2 days. The four bars define the two time intervals zoomed in Figs. 2b and c. b) and c) Zoom of two intervals of Fig. 2a. This (Fig. 2) and all other Figs. 3–7 have been obtained for B ≡ B_∥, ζ = 46, ψ = 40, Δ =  −922.5, β = 0.28, A = 3.84, Φ_r = 0.6, L = 14, and ξ = 1.5 (Sect. 3).