ABSTRACT We report observations which confirm the identity of the optical/IR counterpartto the Rossi X-ray Timing Explorer transient source XTE J0111.2–7317. The coun-terpart is suggested to be a B0-B2 star (luminosity class III–V) showing an IR excessand strong Balmer emission lines. The distance derived from reddening and systemicvelocity measurements puts the source in the SMC. Unusually, the source exhibits anextended asymetric Hα structure.Key words: stars: emission-line, Be - star: binaries - infrared: stars - X-rays: stars -stars: pulsars 1 INTRODUCTIONThe Be/X-ray and supergiant binary systems represent theclass of massive X-ray binaries. A survey of the literaturereveals that of the 96 proposed massive X-ray binary pul-sar systems, 67% of the identified systems fall within theBe/X-ray group of binaries. The orbit of the Be star andthe compact object, presumably a neutron star, is generallywide and eccentric. The optical star exhibits Hα line emis-sion and continuum free-free emission (revealed as excessflux in the IR) from a disk of circumstellar gas. Most of theBe/X-ray sources are also very transient in the emission ofX-rays.Progress towards a better understanding of the physicsof these systems depends on a multi-wavelength programmeof observations. From observations of the Be star in the opti-cal and IR, the physical conditions under which the neutronstar is accreting matter can be determined. In combinationwith hard X-ray timing and flux observations, this yields anear complete picture of the accretion process. It is thusvitalto identify the optical counterparts to these X-ray systemsin order to further our understanding.The X-ray transient XTE J0111.2–7371 was discov-ered by the RXTE X-ray observatory in November 1998(Chakrabarty et al. 1998a,b) as a 31s X-ray pulsar. Thisdetection was confirmed by observations from the BATSEtelescope on the CGRO spacecraft which detected the sourcein the 20–50 keV band. The quick-look results provided by
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