  Synchrotron emission from jets produced by X-ray binaries can be detected at
optical and infrared (IR) frequencies. I show that optical/IR colour-magnitude
diagrams of the outbursts of nine X-ray binaries successfully separate thermal
disc emission from non-thermal jet emission, in both black hole and neutron
star sources. A heated single-temperature blackbody is able to reproduce the
observed relations between colour and magnitude, except when excursions are
made to a redder colour than expected, which is due to jet emission. The
general picture that is developed is then incorporated into the unified picture
of disc-jet behaviour in black hole X-ray binaries. At a given position of a
source in the X-ray hardness-intensity diagram, the radio, IR and optical
properties can be inferred. Similarly, it is possible to predict the X-ray and
radio luminosities and spectral states from optical/IR monitoring.

Astrophysics