  By fitting the frequencies of simultaneous lower and upper kilohertz
Quasi-Periodic Oscillations (kHz QPOs) in two prototype neutron star QPO
sources (4U~1636-536 and Sco X-1), we test the predictive power of all
currently proposed QPO models. Models predict either a linear, power-law or any
other relationship between the two frequencies. We found that for plausible
neutron star parameters (mass and angular momentum), no model can
satisfactorily reproduce the data, leading to very large chi-squared values in
our fittings. Both for 4U~1636-53 and Sco X-1, this is largely due to the fact
that the data significantly differ from a linear relationship. Some models
perform relatively better but still have their own problems. Such a detailed
comparison of data with models shall enable to identify routes for improving
those models further.
