Inner radius of the accretion disc plotted vs. the intrinsic source luminosity for different values of the magnetic field (solid lines), obtained with Eq. (6) (top panel). Derivative of the spin period as a function of luminosity for different values of the magnetic field on a logarithmic scale as indicated in the labels (bottom panel). The dotted lines shown in the plot are the logarithmic of −2.595(11)×10⁻¹² s s⁻¹, i.e. the spin period derivative reported in the literature (Mazzola et al. 2019).