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Astron. Astrophys. 345, L1-L4 (1999)

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3. Discussion

The spectrum of star B is similar to those of massive Be/X-ray binaries. As usual, a more detailed spectral classification is made more difficult by the fact that most of the classical criteria are unusable, since the H and most of the He lines are in emission and the Fe II complex masks most of the stellar atmospheric features. However, the presence of N III, C III and O II lines, and the [FORMULA] 0.1 Å equivalent width of the Mg II 4481 Å line suggest a B0 spectral type (see e.g. Jaschek & Jaschek, 1987). The few absorption lines that are clearly visible are wide, thus suggesting a main sequence star. We can thus conclude that most probably the optical companion of the collapsed object is a B0V star.

The equivalent width [FORMULA]0.5 Å of the interstellar Na II (5890 Å) indicates an intermediate reddening ([FORMULA] [FORMULA] 0.6; following Hobbs, 1974), corresponding, in the Cygnus region to a distance of [FORMULA] 800 pc (Ishida, 1969). We also note that a [FORMULA] [FORMULA] 0.6 translates into a hydrogen column of [FORMULA] 3 [FORMULA] 1021 cm-2 (Zombeck 1990), which is in good agreement with the NH values inferred from the spectral analysis of the merged ROSAT and ASCA data (NH[FORMULA]4 [FORMULA] 1021 cm-2; Israel et al. 1999). This result implies a 1-10 keV X-ray luminosity of 1.2 [FORMULA] 1033 erg s-1. Even though the optical data argue against an accreting white dwarf, only a measurement of the spin period derivative will firmly asses the nature of the accreting object. If the system hosts an accreting neutron star, than it would be one of the faintest Be/X-ray pulsars. Its closest analogoue would be X Per which shows a 1-10 keV X-ray luminosity of 0.7-3.0 [FORMULA] 1034 erg s-1 in its low state (Haberl et al. 1998). Two recently identified Be/X-ray pulsars, namely RX J0440.9+4431/BSD 24-491 and RX J1037.5-564/LS 1698 (Reig & Roche, 1999), also share similar optical and X-ray characteristics with 1WGA J1958.2+3232. If the accreting object is a white dwarf, 1WGA J1958.2+3232 would be the first example of a Be/white dwarf interacting binary system .

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© European Southern Observatory (ESO) 1999

Online publication: April 12, 1999