Long-term modulation and period analysis. a) 8.3 GHz GBI radio data averaged over 3 d. b) Lomb-Scargle analysis results: two frequencies at ν₁ = 0.03775 d^-1 (P₁ = 26.49 d) and ν₂ = 0.03715 d^-1 (P₂ = 26.92 d). The small window shows the peak at 1/1667 d^-1 present in all periodograms. c) Sum of two sinusoidal functions at 26.49 d and 26.92 d, with an amplitude ratio 1/0.7. d) Lomb-Scargle analysis results: two frequencies at and . The significance of the two frequencies becomes identical in the periodogram only for an amplitude ratio 1/1. e) Long-term modulation (1667 d) of a 26.49 d periodic outburst. f) Lomb-Scargle analysis results: one frequency at . The small window to the left shows the peak at P_average present in the periodogram of a simulation of long-term modulation (1667 d) of a 26.70 d periodic outburst. g) Sine wave of periodicity P₁, modulated by a sine wave of periodicity 1667 d and corrected by a sawtooth function. h) Lomb-Scargle analysis results: two frequencies at and as in Figs. 2b and d.