Soft X-ray transients (SXT) are binary star systems formed by a compact object (a black hole or a neutron star) and a low-mass star with a mass some fraction of that of the Sun. Thus, SXTs are a class of low-mass X-ray binaries. SXTs show dramatic changes in their X-ray emission, mainly due to the process of mass transfer from the low-mass donor star to the compact object. This mass accretion process leads to a gradual decrease in the mass of the donor star and to a corresponding increase in the mass of the compact object: in other words, the latter slowly “eats” the former. A strong soft X-ray emission originates from an accretion disk near the compact object. The variations of this soft X-ray emission provide information on how the accretion process occurs . The name of this class of stellar sources comes from this variable X-ray emission. Highly energetic jets of particles and radiation can emanate from the poles of the compact object at nearly the speed of light.