  With the recent commissioning of ground instruments such as SPHERE or GPI and
future space observatories like WFIRST-AFTA, coronagraphy should probably
become the most efficient tool for identifying and characterizing extra-solar
planets in the forthcoming years. Coronagraphic instruments such as Phase mask
coronagraphs (PMC) are usually based on a phase mask or plate located at the
telescope focal plane, spreading the starlight outside the diameter of a Lyot
stop that blocks it. In this communication is investigated the capability of a
PMC to act as a phase-shifting wavefront sensor for better control of the
achieved star extinction ratio in presence of the coronagraphic mask. We
discuss the two main implementations of the phase-shifting process, either
introducing phase-shifts in a pupil plane and sensing intensity variations in
an image plane, or reciprocally. Conceptual optical designs are described in
both cases. Numerical simulations allow for better understanding of the
performance and limitations of both options, and optimizing their fundamental
parameters. In particular, they demonstrate that the phase-shifting process is
a bit more efficient when implemented into an image plane, and is compatible
with the most popular phase masks currently employed, i.e. four-quadrants and
vortex phase masks.
