General relativistic effects in QPOS

Recent results question the applicability of simple Beat Frequency Models, BFMs, to the kHz Quasi Periodic Oscillations, QPOs, of a growing number of Low Mass Xray Binaries, LMXRBs. Yet the evidence that higher frequency QPOs originate from the orbital motion close to the neutron star surface and th e burst oscillation frequency represents the neutron star spin frequency is fairly convincing. We discuss some of the consequences of abandoning BFMs, with special emphasis on the relativistic nodal precession model that has been proposed to explain the low frequency QPOs (similar to 10 - 60 Hz) obs erved in LMXRBs.