Based on observations with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA member states and the USA (NASA).

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Table 1 is only available in electronic form at http://www.edpsciences.org

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...1,

Research Associate FNRS (Belgium).

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... 8.0] keV)

Hereafter we refer respectively to the soft, medium and hard energy bands.

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... 2.2)

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

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... Survey

We used the 2MASS all-sky point source catalogue released in March 2003 and available at http://irsa.ipac.caltech.edu/

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... counterpart

Since the positional errors on the USNO sources are of order 0.3 $^{\prime\prime}$ in both RA and DEC whilst the corresponding errors for the 2MASS catalogue are of order 0.1 $^{\prime\prime}$ , the small positional discrepancies between the optical and IR counterparts are probably not significant.

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... line

Note however that Preibisch & Zinnecker (2002 and references therein) showed that for the young open cluster IC 348 there exists no strict one-to-one relation between PMS stars with IR excesses and cTTs detected through their strong H $\alpha$ emission: while all known cTTs in IC 348 are found to have IR excesses, many wTTs also display IR excess emission.

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...)

The apparent trend of $\log{({L_{\rm X}}/{L_{\rm bol}})}$ with M_V seen in Fig. 8 should not be over interpreted. In fact the X-ray production in the objects at the higher luminosity end (shocks in the winds of B-type stars or unseen low-mass companions?) and those at the fainter end (coronal sources and late-type PMS-like emission) arises most probably through quite distinct physical mechanisms.

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