The identification of the optical/IR counterpart of the 29.5-s transient X-ray pulsar

We report on the identification of the optical/IR counterpart of the 29.5-s transient X–ray pulsar GS 1843+009. We re–analysed an archival ROSAT HRI observation of GS 1843+009 obtaining a new refined position. The optical and IR follow–up observations carried out for the new error circle allowed us to find a relatively faint (V = 20.9) and variable early type reddened star (V − R = 2.1). The optical spectra show the Balmer and Paschen series in emission, while the IR observations confirm the presence of a flux excess (H = 13.2, J−H = 0.54), suggesting that the star is surrounded by a circumstellar envelope. Spectroscopic and photometric data together indicate a B0–B2 IV–Ve spectral–type star located at a distance of ≥ 10 kpc confirming the Be– star/X–ray binary nature of GS 1843+009.