We calculated the evolution of both components of a massive binary system with masses 26 MO and 23.4 MO, initial period 27.4 days, through a case B of mass transfer, up to the end of the core helium burning phase of the primary (and further to the end of the main sequence of the secondary). For the WC phase we adopted Langer's (1989) mass dependent mass loss rate Ṁ ~ M2.5). The luminosities were transformed to visual magnitudes, applying bolometric corrections for O stars of Code et al. (1976) and Flower (1977) and BC = − 4.5 for the WR star (Smith and Maeder, 1989). If conservative mass transfer is adopted, the WR star is about 1.2 to 2.0 magnitude less brighter in the visual than the O star during the WN phase (lasting 1.5 105 yr). The brightness difference reaches almost 3 magnitudes during the WC + 08 phase (lasting 4.3 105 yr), showing the system as a single-lined O-type (!) spectroscopic binary. During the WR phase the mass function of this system varies from 0.67 sin3i to 0.05 sin3i! Nonconservative mass transfer and (or) a smaller initial mass ratio may result in the inverse situation, a single-lined WR binary with a large mass function (De Greve et al., 1988). Some of the results are shown in table 1 below.