Appendices A-C are only available in electronic form at http://www.aanda.org

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There are substantial differences between PMS evolutionary tracks computed by different authors, as discussed e.g. by Baraffe et al. (2002). However, our adoption of the Siess et al. (2000) models does not noticeably affect our results, which are mostly based on comparisons of the colors of NGC 6530 stars with those of normal stars, the latter being taken from the Kenyon & Hartmann (1995) compilation.

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... 0.8 arcsec

We recall that the 0.8 arcsec radius was used to cross-identify the whole optical and IR catalogs, while we conservatively used a smaller radius of 0.4 arcsec for selecting the subset of optical-IR excess stars.

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