| Abstract
| - About 30 per cent of the sources in the fourth INTEGRAL/IBIS catalogue are unidentified in that they lack an optical counterpart. To be able to classify them, X-ray observations are of crucial importance as they can place tighter constraints on the high-energy error box, which is usually of the order of a few arcminutes, and allow their broad-band spectrum to be studied. To this aim we have cross-correlated the list of all unidentified IBIS sources in the fourth catalogue with the archive of all XMM-Newton pointings, finding a set of six objects with archival data. For one of them, IGR J17331−2406, no X-ray source is detected by XMM inside the IBIS error box, most likely due to the fact that it is a transient object. In the case of IGR J17445−2747 two possible X-ray counterparts are found inside the IBIS error box: one is very weak while the other is bright, but only detected once. In each of the remaining four cases: IGR J155359−5750, AX J1739.3−2923, AX J1740.2−2903 and IGR J18538−0102, we find instead a convincing association, for which we provide an improved X-ray position and information on the optical/infrared counterpart. We also performed a detailed analysis of their XMM/IBIS spectra, and on the basis of all information acquired we suggest that IGR J155359−5750 is an active galactic nucleus (AGN) of intermediate type, AX J1739.3−2923 and AX J1740.2−2903 are high-mass X-ray binary systems, IGR J17331−2406 and IGR J17445−2747 are Galactic transient sources and IGR J18538−0102 could be a background AGN.
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