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

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.