MCMC fit for Alessi 1, which has 46 selected MS members. Lower left panels: corner plot of the posterior. The first seven parameters correspond to the polynomial coefficients, p_i, while the last four describe the unresolved multiplicity fraction, f_b, the separation in G magnitude between the binary sequence and the single-star sequence, ΔG, the intrinsic G-magnitude spread of the single-star sequence, σ_SS, and the one of the binary sequence, σ_BS. The main outcome of the fit is the unresolved multiplicity fraction f_b, all the other parameters are considered nuisance parameters. Top right panels: CMD overplotted by 30 random samples of the posterior SS and BS, where the members > 3σ_SS brighter than the fitted SS polynomial are coloured in orange (left). The corresponding residuals with respect to the polynomial fit are also shown as histograms and kernel-density estimates, together with the resulting Gaussian mixture fit (right).