Exo-S (Exo-starshade, for Exoplanet Starshade) is a project to study the concept of a starshade, to aid in the study extra-solar planets via direct imaging. The conceived shade would be used by a telescope launched as some other mission, e.g., WFIRST.
It would include fuel to repeatedly place itself in front of the telescope for observations of targets over some range of the celestial sphere. To make this practical, i.e., do it with a reasonable fuel capacity, movement from one observation to another would be slow, presumably taking weeks.
A parallel study, Exo-C explored a mission to launch a telescope modeled on Kepler, but instrumented for direct imaging, e.g., with a suitable (attached) coronagraph.