  Aims. The wavelet transform has been used as a powerful tool for treating
several problems in astrophysics. In this work, we show that the time-frequency
analysis of stellar light curves using the wavelet transform is a practical
tool for identifying rotation, magnetic activity, and pulsation signatures. We
present the wavelet spectral composition and multiscale variations of the time
series for four classes of stars: targets dominated by magnetic activity, stars
with transiting planets, those with binary transits, and pulsating stars.
Methods. We applied the Morlet wavelet (6th order), which offers high time and
frequency resolution. By applying the wavelet transform to the signal, we
obtain the wavelet local and global power spectra. The first is interpreted as
energy distribution of the signal in time-frequency space, and the second is
obtained by time integration of the local map. Results. Since the wavelet
transform is a useful mathematical tool for nonstationary signals, this
technique applied to Kepler and CoRoT light curves allows us to clearly
identify particular signatures for different phenomena. In particular, patterns
were identified for the temporal evolution of the rotation period and other
periodicity due to active regions affecting these light curves. In addition, a
beat-pattern signature in the local wavelet map of pulsating stars over the
entire time span was also detected.
