Temperature of the circumstellar medium in K at the same time as Figs. 3 and 4. This figure shows the reason for the division in the thin-shell instabilities. At the front of the bowshock as well as between the two stars, the thermalized zone is extremely thin, as a result of effective radiative cooling. Therefore, the shell is subjected to ram-pressure on both sides. In the trailing end (red arrow), the shock is no longer radiative and a relatively thick thermalized zone forms. Hence the zone between contact and termination shock for the LBV wind bubble feels ram-pressure on one side and thermal pressure on the other.