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| - Circumstellar emission in Be/X-ray binaries of the Magellanic Clouds and the Milky Way
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| - Aims. We study the optical and near-infrared colour excesses produced by circumstellar emission in a sample of Be/X-ray binaries. Our main goals are exploring whether previously published relations, valid for isolated Be stars, are applicable to Be/X-ray binaries and computing the distance to these systems after correcting for the effects of the circumstellar contamination. Methods. Simultaneous UBVRI photometry and spectra in the 3500−7000 Å spectral range were obtained for 11 optical counterparts to Be/X-ray binaries in the LMC, 5 in the SMC and 12 in the Milky Way. As a measure of the amount of circumstellar emission we used the H α equivalent width corrected for photospheric absorption. Results. We find a linear relationship between the strength of the H α emission line and the component of E( B − V) originating from the circumstellar disk. This relationship is valid for stars with emission lines weaker than EW ≈ −15 Å. Beyond this point, the circumstellar contribution to E( B − V) saturates at a value ≈0.17 mag. A similar relationship is found for the ( V − I) near infrared colour excess, albeit with a steeper slope and saturation level. The circumstellar excess in ( B − V) is found to be about five times higher for Be/X-ray binaries than for isolated Be stars with the same equivalent width EW(H α), implying significant differences in the physical properties of their circumstellar envelopes. The distance to Be/X-ray binaries (with non-shell Be star companions) can only be correctly estimated by taking into account the excess emission in the V band produced by free-free and free-bound transitions in the circumstellar envelope. We provide a simple method to determine the distances that includes this effect.
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