The discovery of the optical/IR counterpart of the 12-s transient X-ray pulsar GS 0834-43

We report the discovery of the optical counterpart of the 12.3-s transient X-ray pulsar GS 0834-43. We reanalysed archival ROSAT PSPC observations of GS 0834-43, obtaining two new refined positions, similar to 14 and similar to 18 arcsec away from the previously published one, and a new spin period measurement. Based on these results we carried out optical and infrared (IR ) follow-up observations, Within the new error circles, we found a relative ly faint (V = 20.1) early-type reddened star (V - R = 2.24), The optical sp ectrum shows a strong H alpha emission line. The IR observations of the fie ld confirm the presence of an IR excess for the H alpha-emitting star (K' = 11.4, J - K' = 1.94), which is likely surrounded by a conspicuous circumst ellar envelope. Spectroscopic and photometric data indicate a B0-2 V-IIIe s pectral type star located at a distance of 3-5 kpc and confirm the Be-star/ X-ray binary nature of GS 0834-43.