Primary SMBH spin effects: nodal precession. O–C diagrams for the same parameters as in Fig. 7, but where we have considered an almost maximally-rotating Kerr primary SMBH with dimensionless spin of χ = 0.9. The upper panel shows the O–C diagrams over slightly more than one nodal precession period (∼3 yr), while the lower panel are O–C diagrams derived from a restricted 160 d portion of the full light curve, commensurate with the baseline that is probed in GSN 069 by observations between December 2018 and May 2019 (see Fig. 3 and 4).