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 color light curves are analysed simultaneously. Estimated physical parameters of the components are M-1 = 36 3 M-circle dot, M-2 = 10 +/- 1 M-circle dot, R1 = 27 +/- 1 R-circle dot), R-2 = 15 +/- 0.6 R-circle dot, log(L-1/L-circle dot) = 5.59 +/- 0.06, and log(L-2/L-circle dot) = 4.65 +/- 0.07. We give only the formal l 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 +/- 48 pc by using our newly obtained parameters. (C) 2014 Elsevier B.V. All rights reserved.