Interacting O-star binaries: V382 Cyg, V448 CYG and XZ CEP

New spectroscopic observations of three interacting O+O-star binaries (V382 Cyg, V448 Cyg and XZ Cep), obtained at the Dominion Astrophysical Observatory, are analysed in conjunction with published photometry, to yield masses, temperatures and radii. We determine masses of 26.0 +/- 0.7 and 19.3 +/- 0.4 M. for V382 Cyg, 25.2 +/- 0.7 and 14.0 +/- 0.7 M. for V448 Cyg, and 15.8 +/- 0.4 and 6.4 +/- 0.3 M. for XZ Cep.

We find that V382 Cyg is a contact system, resulting from evolutionary expansion of the components during their main-sequence lifetimes (case A mass transfer). V448 Cyg is an evolved binary that has undergone late case A or early case B mass transfer, with the mass loser now filling its Roche lobe and transferring mass more slowly to the gainer, The distance modulus for V448 Cyg of 11.4 agrees well with that of its parent cluster NGC 6871, while the cluster age (7 Myr) places a tight constraint on the evolutionary status of this system, XZ Cep closely resembles V448 Cyg, although its mass is somewhat lower.