  Kilohertz quasi-periodic oscillations (kHz QPOs) are probably caused by
matter in Keplerian orbit at some preferred radius in the accretion disc around
a compact star. In a given source, QPO frequencies can drift by a few hundred
Hz following changes of the inner disc radius; but the disc cannot move closer
to the star than the radius of the innermost stable circular orbit (ISCO)
predicted by general realtivity, hence the kHz QPO frequencies must be limited
by some maximum frequency.
