equal mass binaries with a second likelihood-component identical to the cluster principal curve, but displaced in the magnitude space by 0.75 magnitudes. The relative prior probabilities for the two principal curves were set [*ad hoc*]{} to 0.8 and 0.2 for the main sequence and the sequence of binaries, respectively. These values were not updated during the iterations because the binarity of the sources is not amongst the set of inferred parameters, but also because this fraction varies with stellar luminosity. We checked the consistency of this choice in view of the resulting membership lists in Sect. \[sect:sensitivity\], and discuss alternatives in Sect. \[sect:lim-concl\].

To prevent binaries from biasing the principal curve estimation, we fitted it in two stages. In the first stage, we computed the principal curve for the complete list of members in each iteration of the EM algorithm. This fit is expected to be biased towards brighter magnitudes due to the presence of binaries. We subsequently filtered out sources that are brighter in all magnitudes of the representation space than the closest (in the Euclidean sense) point in the principal curve. We also removed points with Mahalanobis distances from this closest point greater than 15 (a somewhat arbitrary 