An interacting O + O supergiant close binary system: Cygnus OB2-5 (V729 Cyg)

•The radial velocity and light curves of V729 Cyg are solved simultaneously.•The orbital and physical parameters of the system are determined.•The distance of the Cygnus OB2 association is estimated.

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 M1=36±3M⊙,M2=10±1M⊙,R1=27±1R⊙,R2=15±0.6R⊙, log(L1/L⊙)=5.59±0.06log(L1/L⊙)=5.59±0.06, and log(L2/L⊙)=4.65±0.07log(L2/L⊙)=4.65±0.07. We give only the formal 1σ   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±48967±48 pc by using our newly obtained parameters.