Frequency of the highest amplitude peak (ν₀, left panels) and frequency at maximum power (ν_max, right panels) with time for five pure δ Scuti stars with detected resonant mode coupling (one per row; see text). Black circles represent the measurements of each parameter for 20-day segments of the entire light curve. The blue dashed line is the mean value of all 20-d measurements and cyan dotted lines are their error. The red line is the measurement of each parameter for a 1450-day light curve and the orange dashed-dotted lines are their error. The error bars for 20-d measurements of ν₀ are smaller than the symbol. For clarity reasons, we only plotted the error bars for 20-d measurements of ν_max for those outside of the 1450-d measure error.