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 \sim 2.3 \ > kpc .
XTE J1739 - 302 becomes thus characterized as the prototype of the new class of Supergiant Fast X-ray Transients .
The optical and infrared spectra of the counterpart to XTE J1739 - 302 do not reveal any obvious characteristics setting it apart from other X-ray binaries with supergiant companions , which display a very different type of X-ray lightcurve .