tokens: fully convective (e.g., Spada et al. 2017 ). A radius of ≈0.4 R ⊙ leads to cycle times of about 4–5 years, provided that η T remains unchanged. If η T of cooler stars is reduced with respect to the solar value, as implicitly suggested by the stellar models of Spada et al. (2017) , then the cycle time from Eq. ( 1 ) basically exceeds this limit. Mixing-length estimates of 0.33 u rms l corr for M dwarfs provide about 30% of the solar value, so that the dynamo cycle times of M dwarfs should slightly exceed the duration of the solar cycle. Indeed, the cycle time of one star in Fig. 1 slightly exceeds the 11 yr (≈4000 d) limit but this turned out to be a K2 dwarf star with a rotation period of 22.8 d ( Johnson et al. 2016 ). Figure 1 shows the observed cycle lengths for M dwarfs to be generally shorter than ≈3000 d. GJ270 (spectral type M2) shows the longest known cycle period of any M star with 2700 d (7.4 yr), while the shortest cycle is seen in GJ476 (M4) with 1060 d (2.9 yr) ( Robertson et al. 2013a ). GJ328 as an M0 dwarf with 0.69 M ⊙ shows an activity cycle of ≈2000 d, while a Jupiter-mass planet orbits it with a 4100 d period ( Robertson et al. 2013b ). We note that in the solar system the orbital period of Jupiter coincides with the length of the solar activity cycle, accidental or not. GJ581 possesses an entire planetary system with four or possibly six planets and shows an activity cycle with a length of 1600 d ( Gomes da Silva et al. 2012 ; Robertson et al. 2013a ). The rotation period of the latter star is surprisingly long with 130 ± 2 d ( Robertson et al. 2014 ). Fig. 1. Cycle times vs. rotation period (both in days) for early M stars (rhombic symbols). Among the group of rapid rotators with rotation periods less than a day ( Vida et al. 2014 ) are also the early M dwarfs EY Dra and V405 And ( Vida et al. 2013 ). The remaining data are from Suárez Mascareño et al. (2016) ; Wargelin et al. (2017) , and Distefano et al. (2017) . For GJ476 the cycle time is known but not the rotation period (blue bar). The Schwabe cycle of the Sun is indicated as an asterisk denoted SUN. Gomes da Silva et al. (2012) give further examples of M dwarfs with cycle lengths between three and five years. It is of particular importance to know the relation of the cycle time with the rotation time for a homogeneous sample of stars. Savanov (2012) used light curves of cool M dwarfs to find long-term cycle variations up to timescales of years. His sample is mixed by M stars with outer convection zones and also fully convective stars. There seems to be no systematic dependence of the cycle time on the rotation time even for subsamples with slow and fast rotations. An independent study about the coupling of rotation and cycle times for fast-rotating cool stars has been published by Vida et al. (2014) . Among the analyzed 39 Kepler light curves of late-type stars with rotation periods shorter than one day, nine examples showed cyclic variations with periods between
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