The identification of the optical/IR counterpart of the 15.8-s transient X--ray pulsar XTE J1946+274

We report on the discovery of the optical/IR counterpart of the 15.8 s transient X-ray pulsar XTE J1946+274. We re-analyzed archival BeppoSAX observations of XTE J1946+274 obtaining a new refined position (a circle with 22\arcsec radius at 90% confidence level). Based on this new position we carried out optical and infra-red (IR) follow-up observations. Within the new error circle we found a relatively optical faint (B=18.6) IR bright (H=12.1) early type reddened star (V-R=1.6). The optical spectra show strong Halpha and Hbeta emission lines. The IR photometric observations of the field confirm the presence of an IR excess for the Halpha -emitting star (K=11.6, J-H=0.6) which is likely surrounded by a circumstellar envelope. Spectroscopic and photometric data indicate a B0-1V-IVe spectral-type star located at a distance of 8-10 kpc and confirm the Be-star/X-ray binary nature of XTE J1946+274.