  The International Gamma-Ray Astrophyiscs Laboratory (INTEGRAL) is discovering
a large number of new hard X-ray sources, many of them being HMXBs. The
identification and spectral characterization of their optical/infrared
counterparts is a necessary step to undertake detailed study of these systems.
In particular, the determination of the spectral type is crucial in the case of
the new class of Supergiant Fast X-ray Transients (SFXTs), which show X-ray
properties common to other objects. We used the ESO/NTT SofI spectrograph to
observe proposed IR counterparts to HMXBs, obtaining Ks medium resolution
spectra (R = 1320) with a S/N >= 100. We classified them through comparison
with published atlases. We were able to spectrally classify the six sources.
This allowed us to ascribe one of them to the new class of SFXTs and confirm
the membership of two sources to this class. We confirmed the spectral
classification, derived from optical spectroscopy, of a known system, 4U
1907-09, showing for the first time its infrared spectrum. The spectral
classification was also used to estimate the distance of the sources. We
compared the extinction as derived from X-ray data with effective interstellar
extinction obtained from our data, discussing the absorption component due to
the circumstellar environment, which we observed in four systems; in
particular, intrinsic absorption seems to emerge as a typical feature of the
entire class of SFXTs.
