... rates).

A problem was recently discovered in the ML detection code (XMM-Newton News no. 29), which overestimates the ML values by a factor of $\sim$2. Only two of the sources with ML<20 are identified with cluster members (KW 246 and HSHJ 258): a visual inspection of the images shows that KW 246 is a real source, while HSHJ 258 could be dubious. In any case, excluding them from our analysis would not affect our results.

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... database

Operated at CDS, Strasbourg.

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... II

The Guide Star Catalog II is a joint project of the Space Telescope Science Institute and INAF/Osservatorio Astronomico di Torino.

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... catalog

The 2MASS is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology.

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