arXiv:astro-ph/0510675v1 24 Oct 2005 The optical counterpart to the peculiar X-ray transient XTE J1739-302 1 Ignacio Negueruela Dpto. de F´ ısica, Ingenier´ ıa de Sistemas y Teor´ ıa de la Se˜ nal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain ignacio@dfists.ua.es David M. Smith Physics Department and Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, 1156 High St., Santa Cruz, CA 95064 Thomas E. Harrison 2 Astronomy Department, New Mexico State University,Box 30001/Dept. 4500, Las Cruces, NM 88003 and Jos´ eMiguelTorrej´on Dpto. de F´ ısica, Ingenier´ ıa de Sistemas y Teor´ ıa de la Se˜ nal, Universidad de Alicante, Apdo. 99, E03080 Alicante, Spain ABSTRACT 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 ∼ 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. Subject headings: binaries: close — stars: supergiants – X-rays: binaries – X-rays: individual (XTE J1739-302) 1. Introduction The X-ray transient XTE J1739-302 = AX J1739.1-3020 = IGR J17391-3021 is highly un- usual because of its extremely short outbursts. The only outburst that was monitored over its 1 Based on observations collected at the European Southern Observatory, La Silla, Chile (ESO 73.D-0081) 2 Visiting Astronomer at the Infrared Telescope Facil- ity, which is operated by the University of Hawaii under contract from NASA full extent was found to last less than one day (Lutovinov et al. 2005a). In a companion paper (Smith et al. 2005; henceforth Paper I), we provide full characterization of the X-ray behavior of XTE J1739-302 and evidence suggesting that similarly short durations characterize all its outbursts. A pointed Chandra observation led to the un- ambiguous identification of the optical counter- part to XTE J1739-302. This star is cataloged as USNO-B1.0 0596–0585865 = 2MASS 17391155- 1