Distribution of residuals of periods with respect to the fit (solid line in Fig. 2) for close binaries with about equal-mass components M₂/M₁ ≥ 0.9 (top panel) and with non-equal-mass components 0.8 ≤ M₂/M₁ <  0.9 (bottom panel). The top inner plots show the distribution of average ⟨P − P_fit⟩ residuals from Monte Carlo simulations (see text for explanation) with a Gaussian fit over plotted; the bottom inner plots show the distribution of the P − P_fit residuals.