  While jets appear as a fundamental result of accretion processes onto compact
objects in X-ray binaries, there is as yet no standard model for their
underlying physics. The origin of the observed disk-jet coupling also remains
largely unknown. X-ray variability studies have revealed complex variability in
the accretion flow onto stellar-mass black holes and neutron stars, on
timescales as short as milliseconds. The detection of correlated broad-band
rapid time variability in the jet emission would provide valuable information
on how the variability is transferred along the jet and on the timescales of
physical processes operating in these jets, ultimately helping to constrain
internal jet physics, probe disk-jet coupling and infer accretion geometry. In
recent years there have been indirect evidences for optical fast variability
arising from a powerful jet. However, in optical light the emission from the
outer disk can strongly contaminate the jet signal, resulting in an ambiguous
identification of the source of the variability. On the other hand, at much
longer wavelengths the variability, coming from far out in the jet, will be
smeared out in time. Infrared variability studies are thus ideal for looking at
jet variability on the fastest possible timescales. Thanks to newly available
detectors and fast-readout modes, fast infrared and mid-infrared photometry is
now possible. This is opening a new exciting window to study the geometry and
the Physics of relativistic jets and their connection with the accretion flow.
Here we present the first results from a large ongoing fast-timing
multi-wavelength project, showing the first unambiguous evidence for sub-second
jet variable emission. We show how this type of data already allows us to put
quantitative constraints to the jet speed, geometry and physics, and discuss
the great potential of new observations in the near future.
