Response of precession frequencies to changes in NS parameters. Left: Correlations between the lower and upper QPO frequencies, ν_L and ν_U, predicted by the CT and RP models, compared to the observed data from the atoll source 4U 1636−53. The best fits predicted for non-rotating NSs are denoted by thick solid lines, while the best fits for moderately rotating NSs with j = 0.22 and q = 6j² are denoted by dashed lines. The thin solid lines show these values of M and j but different values of q (q = 1.5j²). For the CT model, the light-coloured region emphasises the change in the frequencies predicted by the model with changing q. Middle: Relative change in the cusp torus precession frequencies with angular momentum at the radius where ν_RP = 0.75 ν_K. Within the RP and CT models, this radius corresponds to the bottom-left group of data points shown in the left panel. The light-coloured region emphasises the corresponding change in ν_RP. Right: Relative change in the precession frequencies with the quadrupole parameter. The frequencies are calculated at the same radius as in the middle panel.