The optical counterpart to the peculiar X-ray transient XTE J1739-302

The weak X-ray transient XTE J1739 - 302, characterized by extremely short outbursts, has recently been identified with a reddened star. Here we present spectroscopy and photometry of the counterpart, identifying it as a O8 Iab(f) supergiant at a distance of similar to 2.3 kpc. XTE J1739 - 302 thus becomes the prototype of the new class of supergiant fast X-ray transients (SFXTs). The optical and infrared spectra of the counterpart to XTE J1739 - 302 do not reveal any obvious characteristics setting it apart fromother X-ray binaries with supergiant companions, which display a very different type of X-ray light curve.