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The long-term (over more than one decade) X-ray emission from two massivestellar systems known to be particle accelerators is investigated usingXMM-Newton. Their X-ray properties are interpreted taking into account recentinformation about their multiplicity and orbital parameters. The two targets,HD168112 and HD167971 appear to be overluminous in X-rays, lending additionalsupport to the idea that a significant contribution of the X-ray emission comesfrom colliding-wind regions. The variability of the X-ray flux from HD168112 isinterpreted in terms of varying separation expected to follow the 1/D rule foradiabatic shocked winds. For HD167971, marginal decrease of the X-ray flux inSeptember 2002 could tentatively be explained by a partial wind eclipse in theclose pair. No long-term variability could be demonstrated despite thesignificant difference of separation between 2002 and 2014. This suggests thecolliding-wind region in the wide orbit does not contribute a lot to the totalX-ray emission, with a main contribution coming from the radiative shockedwinds in the eclipsing pair. The later result provides evidence that shocks ina colliding-wind region may be efficient particle accelerators even in theabsence of bright X-ray emission, suggesting particle acceleration may operatein a wide range of conditions. Finally, in hierarchical triple O-type systems,thermal X-rays do not necessarily constitute an efficient tracer to detect thewind-wind interaction in the long period orbit.

Long-term XMM-Newton investigation of two particle-accelerating colliding-wind binaries in NGC6604: HD168112 and HD167971