  Aims: To gain a better understanding of high-energy Galactic sources, we
observed the Galactic X-ray binary and black hole candidate XTE J1720-318 in
the optical and near-infrared, just after the onset of its X-ray outburst in
January, 2003. These observations were obtained with the ESO/NTT as the Target
of Opportunity, in February and April 2003. Methods: We performed an accurate
astrometry and analysed photometrical and spectroscopic observations. We then
produced a colour-magnitude diagram, looked at the overall evolution of the
multi-wavelength light curve, and analysed the spectral energy distribution.
Results: We discovered the optical counterpart in the R-band (R ~ 21.5) and
confirmed the near-infrared counterpart. We show that, for an absorption
between 6 and 8 magnitudes, XTE J1720-318 is likely to be an intermediate mass
X-ray binary located at a distance between 3 and 10 kpc, hosting a main
sequence star of spectral type between late B and early G. Our second set of
observations took place simultaneously with the third secondary outburst
present in X-ray and near-infrared light curves. The evolution of its spectral
energy distribution shows that XTE J1720-318 entered a transition from a
high-soft to a low-hard state in-between the two observations.
