  The long-period O-star binary system HD 168112 and the triple O-star system
HD 167971 are well-known sources of non-thermal radio emission that arises from
a colliding wind interaction. The wind-wind collisions in these systems should
result in phase-dependent X-ray emissions. The presence of a population of
relativistic electrons in the wind interaction zone could affect the properties
of the X-ray emission and make it deviate from the behaviour expected for
adiabatic shocks. We investigate the X-ray emission of these systems with the
goals of quantifying the fraction of the X-ray flux arising from wind
interactions and determining whether these emissions follow the predictions for
adiabatic wind-wind collisions. Six X-ray observations were collected with
XMM-Newton. Three observations were scheduled around the most recent periastron
passage of HD 168112. Spectra and light curves were analysed and compared with
simple predictions of model calculations for X-ray emission from colliding wind
systems. The X-ray emission of HD 168112 varies as the inverse of the orbital
separation, as expected for an adiabatic wind interaction zone. The relative
contribution of intrinsic X-ray emission from wind-embedded shocks varies
between 38% at periastron to 81% at apastron. The wind-wind collision zone
remains adiabatic even around periastron passage. The X-ray emission of HD
167971 displays variations on the orbital timescale of the inner eclipsing
binary. The existing data of this system do not allow us to probe variations on
the timescale of the outer orbit. Shock modification due to the action of
relativistic electrons does not seem to be efficiently operating in the HD
168112 system. In the existing observations, a significant part of the emission
of HD 167971 must arise in the inner eclipsing binary. The origin of this
emission is as yet unclear.
