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.
New Worlds Mission