Based on observations carried out at the European Southern Observatory, La Silla, Chile under proposals numbers 67.C-0225 and 68.C-0311.

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Appendices and Fig. 14 are only available in electronic form at http://www.aanda.org

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...

Table 7 is available in electronic form at http://www.aanda.org and at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/470/281

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... IRAF

IRAF is distributed by the National Optical Astronomy Observatories (NOAO).

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... tool

Available at: http://www.na.astro.it/radovich

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... tool

Available at: http://terapix.iap.fr/cplt/oldSite/soft/swarp

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... bands

Table 7 is published only in electronic form.

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...López-Martí et al. (2005)

The BD candidates selected by López-Martí et al. (2005) do not comply with our selection criteria, probably because the colour effects on the isochrones due the WFI filters were not taken into account.

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... $M_{\odot}$

Luminosity and temperature determinations are reported by Luhman (2004) for 144 members of Cha I. We derived masses and ages for these objects using the evolutionary tracks by Baraffe et al. (1998) and Chabrier et al. (2000) in a homogeneous way.

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... curve

Available at: http://www.ls.eso.org/lasilla/sciops/2p2/E2p2M/WFI/filters

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