  The X-ray emission of the O+O binary HD166734 was monitored using Swift and
XMM-Newton observatories, leading to the discovery of phase-locked variations.
The presence of an f line in the He-like triplets further supports a wind-wind
collision as the main source of the X-rays in HD166734. While temperature and
absorption do not vary significantly along the orbit, the X-ray emission
strength varies by one order of magnitude, with a long minimum state
(Delta(phi)~0.1) occurring after a steep decrease. The flux at minimum is
compatible with the intrinsic emission of the O-stars in the system, suggesting
a possible disappearance of colliding wind emission. While this minimum cannot
be explained by eclipse or occultation effects, a shock collapse may occur at
periastron in view of the wind properties. Afterwards, the recovery is long,
with an X-ray flux proportional to the separation d (in hard band) or to d^2
(in soft band). This is incompatible with an adiabatic nature for the collision
(which would instead lead to "F_X propto 1/d"), but could be reconciled with a
radiative character of the collision, though predicted temperatures are lower
and more variable than in observations. An increase in flux around phi~0.65 and
the global asymmetry of the light curve remain unexplained, however.
