  We report on the first detection of a global change in the X-ray emitting
properties of a wind-wind collision, thanks to XMM-Newton observations of the
massive SMC system HD5980. While its lightcurve had remained unchanged between
2000 and 2005, the X-ray flux has now increased by a factor of ~2.5, and
slightly hardened. The new observations also extend the observational coverage
over the entire orbit, pinpointing the lightcurve shape. It has not varied much
despite the large overall brightening, and a tight correlation of fluxes with
orbital separation is found, without any hysteresis effect. Moreover, the
absence of eclipses and of absorption effects related to orientation suggests a
large size for the X-ray emitting region. Simple analytical models of the
wind-wind collision, considering the varying wind properties of the eruptive
component in HD5980, are able to reproduce the recent hardening and the
flux-separation relationship, at least qualitatively, but they predict a
hardening at apastron and little change in mean flux, contrary to observations.
The brightness change could then possibly be related to a recently theorized
phenomenon linked to the varying strength of thin-shell instabilities in
shocked wind regions.
