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The massive interacting close binary system V729 Cyg (OIa+O/WN9), plausiblyprogenitor of a Wolf-Rayet system, is studied using new observations gatheredover 65 nights and earlier published data. Radial velocity and five colourlight curves are analysed simultaneously. Estimated physical parameters of thecomponents are $M_1 = 36\pm 3 M_{\odot}$, $M_2 = 10 \pm 1 M_{\odot}$, $R_1 =27\pm 1 R_{\odot}$, $R_2 = 15 \pm 0.6 R_{\odot}$, $\log{(L_1/L_{\odot})} =5.59\pm 0.06$, and $\log{(L_2/L_{\odot})} = 4.65\pm 0.07$. We give only theformal 1$\sigma$ scatter, but we believe systematic errors in the luminosities,of uncertain origin as discussed in the text, are likely to be much bigger. Thedistance of the Cygnus OB2 association is estimated as $967\pm 48$ pc by usingour newly obtained parameters.

An Interacting O+O Supergiant Close Binary System: Cygnus OB2-5 (V729 Cyg)