... interactions

Based on observations collected at the European Southern Observatory, La Silla, Chile.

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...1,

Research Associate FNRS, Belgium.

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...1,

Research Fellow FNRS, Belgium.

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... spectrum

We note that this value is likely to be a lower limit on the actual line strength because of the contamination of the blend by several DIBs.

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... solution

Bonanos et al. already recalculated an orbital solution, adopting a priori a single value for the systemic velocity of both stars. Since the lines measured to evaluate the radial velocities form in moving stellar winds, this could in principle bias the amplitude of the orbital solution.

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...$M_{\odot }$

All error bars in this paper correspond to 1$\sigma$ uncertainties.

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... system

Given the lack of information on magnetic fields of early-type stars, we do not consider the impact of magnetic fields on the interaction in WR 20a.

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... index

The (b - v)₀ index refers to the Smith (1968) ubvr photometric system. This system was designed for Wolf-Rayet stars and is based on a set of essentially emission line-free narrow band filters. The intrinsic (b - v)₀ colour index of five WN6ha stars studied by Crowther & Dessart (1998) amounts to $-0.29 \pm 0.02$.

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... Table 27)

The latter result refers again to the Smith (1968) ubvr photometric system. We have corrected the absolute magnitude given in Table 27 of van der Hucht (2001) for the revised distance of NGC 3603 by Sagar et al. (2001) compared to Crowther & Dessart (1998).

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