An explanation for the kHz-QPO twin peaks separation in slow and fast   rotators
  In this Letter we further explore the idea, suggested previously by Klu{\'z}niak and collaborators, that the high frequency QPOs may be explained as a resonant coupling between the neutron star spin and two epicyclic modes of accretion disk oscillations. We confirm result of Lee, Abramowicz and Klu{\'z}niak (\cite{Lee2004}) that the strongest response occurs when the frequency difference of the two modes equals either the spin frequency (for ``slow rotators'') or half of it (for ``fast rotators''). New points discussed in this Letter are: (1) We suggest that the coupling is gravitational, and due to a non-axially symmetric structure of the rotating neutron star. (2) We found that two excited modes may be both connected to vertical oscillations of the disk, and that strong gravity is not needed to excite the modes. 
J. Petri