Binary stars mass-ratio as a function of mass-ratio in NGC 2099. Reproduction of the CMD of NGC 2099 plotted in Fig. 8b (panel a). The shaded regions (B1, B2, and B3) of the CMD comprise binaries with different mass ratios. The candidate binaries are marked with white crosses. Panel b shows the mass-ratio distribution of binaries in NGC 2099. The χ² and the probability that the observed binary fractions come from a flat mass-ratio distribution are quoted in panel b.