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 re–analysed archival ROSAT PSPC observations of GS 0834–43 obtaining two new refined positions, ∼14 and ∼18 away from the previously published one, and a new spin period measurement. Based on these results we carried out optical and infra–red (IR) follow–up observations. Within the new error circles we found a relatively faint (V=20.1) early type reddened star (V–R=2.24). The optical spectrum shows a strong Hα emission line. The IR observations of the field confirm the presence of an IR excess for the Hα–emitting star (K=11.4, J–K=1.94) which is likely surrounded by a conspicuous circumstellar envelope. Spectroscopic and photometric data indicate a B0–2 V–IIIe spectral–type star located at a distance of 3–5kpc and confirm the Be–star/X–ray binary nature of GS 0834–43.