Variation in the large separation with frequency, derived from the autocorrelation of the time series. The power spectrum was first windowed with a 0.75 mHz-half width cosine filter, as indicated in the lower-right corner. The sampling of the curve corresponds crudely to the mean large separation. The size of the dots is representative of the amplitude of the correlated signal. One-σ error bars (dashed lines) are derived from Mosser & Appourchaux (2009).