Amplitude and frequency variations of the linear combination frequency f₂₃ = f₁ + f₁₁. Top panel: measured amplitudes as a function of time obtained from each data subset (using the same method as for multiplets). Middle panel: measured frequencies from each data subset compared with the frequency sum f₁ + f₁₁, both as a function of time. Bottom panel: observed amplitudes of f₂₃ vs predicted amplitudes of R × the product of f₁ and f₁₁ amplitudes (see Eq. (6) for the definition of R). In both cases (frequency and amplitude comparisons), the measurements are found to be within 1σ.