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Context: The INTEGRAL hard X-ray observatory has revealed an emergingpopulation of highly obscured X-ray binary systems through multi-wavelengthobservations. Previous studies have shown that many of these sources arehigh-mass X-ray binaries hosting neutron stars orbiting around luminous andevolved companion stars. Aims: To better understand this newly-discoveredpopulation, we have selected a sample of sources for which an accuratelocalisation is available to identify the stellar counterpart and reveal thenature of the companion star and of the binary system. Methods: We performed anintensive study of a sample of thirteen INTEGRAL sources, throughmulti-wavelength optical to NIR photometric and spectroscopic observations,using EMMI and SofI instruments at the ESO NTT telescope. We performed accurateastrometry and identified candidate counterparts for which we give the opticaland NIR magnitudes. We detected many spectral lines allowing us to determinethe spectral type of the companion star. We fitted with stellar black bodiesthe mid-infrared to optical spectral energy distributions of these sources.From the spectral analysis and SED fitting we identified the nature of thecompanion stars and of the binary systems. (abridged).

Multi-wavelength observations of Galactic hard X-ray sources discovered by INTEGRAL. I. The nature of the companion star