Correlation between logarithm of the cycle period and the colour index B − V, relative depth of the convection zone and convective turnover time. Left panels: Logarithm of the cycle period versus the colour index B − V in panel a, versus the relative depth of the convection zone in panel b, and versus the convective turnover time in panel c. Right panels: Difference between the logarithm of the cycle period and the logarithm of the rotation period versus the colour index B − V is shown in panel d, versus the relative depth of the convection zone in panel e and the convective turnover time in panel f. The red solid line in panels d to e shows the estimated trend between both values; see text for details.