Normalized histogram of all flux ratios, log (F₁/F₂), taken between 30 ks time intervals separated by Δt = 7.1–9.6 days (logarithmic mean: 8.24 days), for the 167 Class III stars in our flux-limited sample (broken line). The narrower distribution, actually a bundle of ten almost indistinguishable ones, represents the contribution of Poisson statistics to the observed distribution as determined from Monte Carlo simulations (see text).