Based on observations collected at the Observatorio del Teide (Tenerife, Spain).

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The complete Fig. 1 is available in electronic form at http://www.edpsciences.org and Table 2 is only available in electronic form 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/375/539

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... $H-K \simeq 4.2$

We prefer to give in our descriptions of individual objects the K magnitude, which is generally determined with better precision than H, especially in the case of very red sources. However, since the latter is expected to be less affected by circumstellar emission, we have preferred to use it in the vertical axis of the color-magnitude diagrams.

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