Precessing period due to the interaction of the NS magnetic field with the accretion disc (Lipunov & Shakura 1980) as a function of the NS spin period calculated for different values of the NS dipole magnetic field. Left and right panels: calculations for ϵ_w = 0.2 and ϵ_w = 0.8, respectively. The horizontal dashed line indicates the super-orbital period of 110 days of M 81 X-6 (Lin et al. 2015), and we have considered the temperature of the soft component in the calculations of R_mag.