The wind-wind collision region of the Wolf-Rayet binary V444 Cygni: How much optical line emission does it produce?

We model the emission-line profile variations that are expected to be produced by physical and wind eclipses in the Wolf-Rayet (W-R + O) binary system V444 Cyg. A comparison of the theoretical profiles with the He II 4686 Angstrom line observed in V444 Cyg allows us to isolate the effects that are likely to be due to the wind-wind collision region in this particular line. We estimate that the wind-wind collision region contributes no more than similar to 12% of the equivalent width of the emission line, with smaller values during elongations, when part of the shock cone is being eclipsed by the O star. The upper limit implies a maximum contribution from the wind-wind collision region of similar to1 x 10(35) ergs s(-1) to the total luminosity of He II 4686 Angstrom line. Using the analytical solution of Canto et al., we find that the bulk of this emission arises along the shock cone walls where the flow velocity is similar to 800 km s(-1), at a distance of similar to8 R. from the O star's surface, and at theta = 65 degrees -75 degrees from the line joining the centers of the two stars, with origin in the O star. The derived surface density of this region is sigma = 0.22 g cm(-2), which, together with the He II 4686 Angstrom luminosity, indicates that the thickness of the shock lies in the range 2-10 x 10(10) cm and the total density is 1-6 x 10(12) cm(-3).