2012A&A...539A.114R

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. Simultaneous UBVRI photometry and spectra in the 3500-7000Å 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. 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≃-15Å. Beyond this point, the circumstellar contribution to E(B-V) saturates at a value ≃0.17mag. 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.