Looking for activity cycles in late-type Kepler stars using time-frequency analysis

We analyse light curves covering four years of 39 fast-rotating (P-rot a parts per thousand(2) 1 d) late-type active stars from the Kepler data base. Using time-frequency analysis (short-term Fourier transform), we find hints for activity cycles of 300-900 d at nine targets from the changing typical latitude of the starspots, which with the differential rotation of the stellar surface change the observed rotation period over the activity cycle. We also give a lowest estimation for the shear parameter of the differential rotation, which is a parts per thousand 0.001 for the cycling targets. These results populate the less studied, short-period end of the rotation-cycle length relation.