  We report on several European Southern Observatory (ESO) near-infrared (NIR)
observational campaigns aimed at understanding the nature of galactic black
hole candidates. Our results, including NIR photometry of the sources GRO
J1655-40, GRS 1739-278, GRS 1716-249, GRS 1121-68 and GX 339-4, show that all
the sources but GRO J1655-40 are consistent with low-mass stars as the
companion star of the binary system.
  By locating the counterparts on a colour-magnitude diagram (CMD), we better
constrain the spectral type of the companion star of three of the systems
considered here, and confirm a fourth one. The spectral types are respectively:
M0-5 V for GRS 1716-249, F8-G2 III for GX 339-4 and later than F5 V for GRS
1739-278. We confirm the already known spectral type of the companion in GRS
1121-68 (K0-5 V). The location of GRO J1655-40 on the CMD is consistent with
the sub-giant luminosity class and with this source crossing the Hertzsprung
gap. However, a non-stellar emission seems to contribute to the NIR flux of
this source.
