  Using XMM-Newton, we undertook a dedicated project to search for X-ray bright
wind-wind collisions in 18 WR+OB systems. We complemented these observations
with Swift and Chandra datasets, allowing for the study of two additional
systems. We also improved the ephemerides, for these systems displaying
photometric changes, using TESS, Kepler, and ASAS-SN data. Five systems
displayed a very faint X-ray emission ($\log [L_{\rm X}/L_{\rm BOL}]<-8$) and
three a faint one ($\log [L_{\rm X}/L_{\rm BOL}]\sim-7$), incompatible with
typical colliding wind emission: not all WR binaries are thus X-ray bright. In
a few other systems, X-rays from the O-star companion cannot be excluded as
being the true source of X-rays (or a large contributor). In two additional
cases, the emission appears faint but the observations were taken with the WR
wind obscuring the line-of-sight, which could hide a colliding wind emission.
Clear evidence of colliding winds was however found in the remaining six
systems (WR19, 21, 31, 97, 105, 127). In WR19, increased absorption and larger
emission at periastron are even detected, in line with expectations of
adiabatic collisions.

Solar and Stellar Astrophysics;High Energy Astrophysical Phenomena