Left: Normalised mean standard deviation as a function of k, which is the multiplicative factor of P _rot to determine the size of the subseries, for the 34 M dwarfs of our sample (black curves) and the Sun (red curve). Right: Magnetic activity index for the value k = 5, , corrected from the photon noise, as a function of the rotation period for the 31 non-polluted M dwarfs. The red dashed line corresponds to the solar value.