Density of the circumstellar medium in the LBV + O system at the same moment in time as Fig. 3. This figure shows a cross-section in the orbital plane of the binary. The wind from the luminous blue variable (blue sphere) dominates because of its higher ram-pressure. The shell created by the collision is unstable, showing two kinds of thin shell instabilities. Non-linear thin-shell instabilities occur at the head of the bow shock around the O-star (white sphere) and in the region directly between the two stars. Downstream (red arrow), the instabilities are of linear thin-shell type. Even further downstream, after the shell has made formed a curve around the approaching LBV star the shell begins to diffuse because of its own internal pressure.