The study of fast variability in X-ray binaries is one of the most powerful tools to study the accretion flow in the vicinity of compact
objects. In particular, thanks to newly available detectors, fast OIR photometry is now opening a new exciting window to study the geometry and the
physics of accretion. I will review the main properties of the low-frequency variability observed in accreting black holes and neutron stars, as well as
the potentialities of coordinated fast multiwavelength photometry. I will present some preliminary results from an ongoing multiwavelength project, and
will discuss possible physical interpretations, focusing on the important role the relativistic jet might play.