The identification of the optical/IR counterpart of the 29.5-s transient X-ray pulsar GS 1843+009

We report on the identification of the optical/IR counterpart of the 29.5-s transient X-ray pulsar GS1843+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 greater than or equal to 10 kpc confirming the Be-star/X-ray binary nature of GS 1843+009.