The H ${\alpha }$ results presented here are based on observations made with ESO Telescopes at the La Silla or Paranal Observatories under program IDs 67.D-0229(A), 69.D-0628(A) and 075.D-0591(A); the Observatoire de Haute-Provence; the Observatório do Pico dos Dias / LNA, Brazil; the G. D. Cassini telescope operated at the Loiano Observatory by the Osservatorio Astronómico di Bologna; and the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias (those data were taken using ALFOSC, which is owned by the Instituto de Astrofísica de Andalucía (IAA) and operated at the Nordic Optical Telescope under agreement between IAA and the NBIfAFG of the Astronomical Observatory of Copenhagen).

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Table 2 is only available in electronic form at http://www.aanda.org

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... inclination

We note that throughout this work, the errors in phases -not shown- range between 0.03-0.04 due to the uncertainties in T₀ and $P_{\rm orb}$ .

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... FEROS

All FEROS observations were obtained during the Brazilian time allocation.

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...Å)

These has been obtained averaging few months data around the XMM-Newton observation dates, which implies to assume a steady H ${\alpha }$ behavior at these timescales.

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...1995)

To obtain a conservative error for the $N_{\rm H}-A_{\rm V}$ relationship given in Predehl & Schmitt (1995), we use the dispersion and not the error of the mean.

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... system

Such special wind structure may be produced by the slow compact object motion through the wind, although it can hardly have strong impact since the supersonic wind will efficiently convect outwards any possible perturbation.

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