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.
The plan is to give it the fuel to place itself in front of the telescope for each observation of some coordinate. 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 coronagraph.