Top left panel: proper motions of stars in field of NGC 752. The colour code corresponds to the groups identified by k-means clustering in the (μ_α*,  μ_δ,  ϖ) space. The sky distribution of the three highlighted groups is shown in the other panels. The quantity Λ is a measurement of spatial clustering, as defined in Eq. (1). Stars are considered potential cluster members when Λ > 1.