  The correlation between the frequency and the absolute value of the frequency
derivative of the kilohertz QPOs observed for the first time from 4U 1636-53 is
a simple consequence and indicator of the existence of a non-Keplerian rotation
rate in the accretion disk boundary layer. This paper interprets the observed
correlation, showing that the observations provide strong evidence in support
of the fundamental assumption of disk accretion models around slow rotators,
that the boundary layer matches the Keplerian disk to the neutron star
magnetosphere.
