Comparison between the upper QPO frequencies predicted by the Ep and RP0 models. Left: examples of the non-trivial topology of ν_U(β) curves predicted by the Ep model. Except for the a = 0 case, we do not display the whole effective range of β ∈ [0,  β_cusp]; the overall increase of ν_U is rather small for any a ≲ 0.9. Right: monotonic behaviour of ν_U(β) functions predicted by the RP0 model. The values of a are the same as in the left panel. In both panels, the red dots correspond to β = β_cusp. The empty red circle denotes β = β_∞.