  Our knowledge of magnetic activity of stars is mostly based on the study of
the Sun and some spectroscopic surveys of a few hundreds of stars. However, the
detailed mechanisms of the solar magnetic activity cycle are not fully
understood. With the Kepler mission that is providing exquisite photometric
data for hundred of thousands of stars monitored continuously for more than
three year, we can study changes in the light curves that can be related to
magnetic activity. We show here a few examples of stars for which we did a
time-frequency analysis to look for such signature.
