A large collection of blue and yellow CCD spectra of the short-period eclipsing binary CQ Cep allows us to improve the double-line (SB2) orbit of Kartasheva and Snezhko and reclassify the system as WN6+O9 II-Ib. With orbital inclination i = 78 degrees-65 degrees, the masses and radii are M(O) = 18-23 M(.), M(WR) = 15-19 M(.), R(O) less than or equal to 10 R(.), 2 R(.) less than or equal to R(WR) less than or equal to 10 R(.). For its spectral type, the O star mass and radius are smaller than values deduced from evolutionary models but agree with recent determinations of other O stars in binaries. The W-R star is shown to contain a small amount of hydrogen in its wind. Because of the rapid orbit revolution compared to the wind velocities, the wind interaction zone is tightly wrapped around the system. We construct a phenomenological model which explains the numerous phase-dependent spectral variations.
