  Recently, evidence for synchrotron emission in both black hole and neutron
star X-ray binaries has been mounting, from optical/infrared spectral,
polarimetric, and fast timing signatures. The synchrotron emission of jets can
be highly linearly polarised, depending on the configuration of the magnetic
field. Optical and infrared (OIR) polarimetric observations of X-ray binaries
are presented in this brief review. The OIR polarimetric signature of
relativistic jets is detected at levels of ~ 1-10 %, similar to AGN cores. This
reveals that the magnetic geometry in the compact jets may be similar for
supermassive and stellar-mass BHs. The magnetic fields near the jet base in
most of these systems appear to be turbulent, variable and on average, aligned
with the jet axis, although there are some exceptions. These measurements probe
the physical conditions in the accretion (out)flow and demonstrate a new way of
connecting inflow and outflow, using both rapid timing and polarisation.
Variations in polarisation could be due to rapid changes of the ordering of the
magnetic field in the emitting region, or in one case, flares from individual
ejections or collisions between ejecta. It is predicted that in some cases,
variable levels of X-ray polarisation from synchrotron emission originating in
jets will be detected from accreting Galactic black holes with upcoming
spaceborne X-ray polarimeters.
