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 a system. 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.