  X-ray binaries are binary star systems in which a compact object (a neutron
star or a black hole) and a relatively normal star orbit a common centre of
mass. Since the discovery of X-ray binaries with the first X-ray telescopes in
the 1960s, astronomers have tried to understand how these bizarre objects
behave, and why. Some change in X-ray luminosity by 10^8 orders of magnitude on
timescales of days to months due to an increased transfer of mass from the star
towards the compact object. Many X-ray binaries are detected at all observable
frequencies, from radio to gamma-rays. It has been found that many different
sources of emission, which peak at different frequencies, are present in X-ray
binary spectra and together they produce the observed broadband spectrum.
However, disentangling these components has proved challenging. Much of the
work in this thesis concerns disentangling the components that occupy the
optical and near-infrared (NIR) region of the spectrum of X-ray binaries;
possibly the region in which the relative contributions of the different
components are least certain. In particular one component, the synchrotron
emission from jets of outflowing matter, is found in this work to contribute
ubiquitously to the optical and NIR light of X-ray binaries with relatively
faint stars. These results confirm that the jets are powerful and in some of
this work, observations of the jets interacting with the surrounding matter are
used to infer their power.
