  The study of stellar activity is important because it can provide new
constraints for dynamo models, when combined with surface rotation rates and
the depth of the convection zone. We know that the dynamo mechanism, which is
believed to be the main process to rule the magnetic cycle of solar-like stars
at least, results from the interaction between (differential) rotation,
convection, and magnetic field. The Kepler mission has been collecting data for
a large number of stars during 4 years allowing us to investigate magnetic
stellar cycles. We investigated the Kepler light curves to look for magnetic
activity or even hints of magnetic activity cycles. Based on the photometric
data we also looked for new magnetic indexes to characterise the magnetic
activity of the stars. We selected a sample of 22 solar-like F stars that have
a rotation period smaller than 12 days. We performed a time-frequency analysis
using the Morlet wavelet yielding a magnetic proxy. We computed the magnetic
index S_ph as the standard deviation of the whole time series and the index
<S_ph> that is the mean of standard deviations measured in subseries of length
five times the rotation period of the star. We defined new indicators to take
into account the fact that complete magnetic cycles are not observed for all
the stars, such as the contrast between high and low activity. We also inferred
the Rossby number of the stars and studied their stellar background. This
analysis shows different types of behaviours in the 22 F stars. Two stars show
behaviours very similar to magnetic activity cycles. Five stars show long-lived
spots or active regions suggesting the existence of active longitudes. Two
stars of our sample seem to have a decreasing or increasing trend in the
temporal variation of the magnetic proxies. Finally the last group of stars
show magnetic activity (with presence of spots) but no sign of cycle.
