  Recently, the study of the CoRoT target HD 49933 showed evidence of
variability of its magnetic activity. This was the first time that a stellar
activity was detected using asteroseismic data. For the Sun and HD 49933, we
observe an increase of the p-mode frequencies and a decrease of the maximum
amplitude per radial mode when the activity level is higher. Moreover a similar
behavior of the frequency shifts with frequency has been found between the Sun
and HD 49933. We study 3 other targets of CoRoT as well, for which modes have
been detected and well identified: HD 181420, HD 49385, and HD 52265 (which is
hosting a planet). We show how the seismic parameters (frequency shifts and
amplitude) vary during the observation of these stars.
