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Given the closeness of the two open clusters Cr 135 and UBC 7 on the sky, weinvestigate the possibility of the two clusters to be physically related. Weaim to recover the present-day stellar membership in the open clustersCollinder 135 and UBC 7 (300 pc from the Sun), to constrain their kinematicparameters, ages and masses, and to restore their primordial phase spaceconfiguration. The most reliable cluster members are selected with ourtraditional method modified for the use of Gaia DR2 data. Numerical simulationsuse the integration of cluster trajectories backwards in time with our originalhigh order Hermite4 code \PGRAPE. We constrain the age, spatial coordinates andvelocities, radii and masses of the clusters. We estimate the actual separationof the cluster centres equal to 24 pc. The orbital integration shows that theclusters were much closer in the past if their current line-of-sight velocitiesare very similar and the total mass is more than 7 times larger the mass of thedetermined most reliable members. We conclude that the two clusters Cr 135 andUBC 7 might very well have formed a physial pair, based on the observationalevidence as well as numerical simulations. The probability of a chancecoincidence is only about $2\%$.

Collinder 135 and UBC 7: A Physical Pair of Open Clusters