Based on observations obtained at the 2.5 m INT telescope of the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. This research has also made use of the SIMBAD database, operated at CDS, Strasbourg, France.

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

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... widths

According to the standard definition, the equivalent width of an emission line would be negative. However, in order to simplify the discussion, in this paper we use a positive sign, so that a larger H $\alpha$ equivalent width corresponds to a stronger H $\alpha$ emission, and to a larger r-H $\alpha$ colour.

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