Rms deviation σ_model of the Keplerian fit with P = 1 from the 2:1 resonant input model (solid line with points and labels). This deviation is expressed in units of the RV semi-amplitude K and plotted as a function of the eccentricity e determined by the fit. The labels indicate the amplitude ratio of the input sinusoids. Thin dashed line: amplitude of the first harmonic in the Fourier expansion of the Kepler equation.