  The massive interacting close binary system V729 Cyg (OIa+O/WN9), plausibly
progenitor of a Wolf-Rayet system, is studied using new observations gathered
over 65 nights and earlier published data. Radial velocity and five colour
light curves are analysed simultaneously. Estimated physical parameters of the
components 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 the
formal 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. The
distance of the Cygnus OB2 association is estimated as $967\pm 48$ pc by using
our newly obtained parameters.

Solar and Stellar Astrophysics