  Quantized orbital structures are typical for many aspects of classical
gravity (Newton's as well as Einstein's). The astronomical phenomenon of
orbital resonances is a well-known example. Recently, Rothman, Ellis and
Murugan (2001) discussed quantized orbital structures in the novel context of a
holonomy invariance of parallel transport in Schwarzschild geometry. We present
here yet another example of quantization of orbits, reflecting both orbital
resonances and holonomy invariance. This strong-gravity effect may already have
been directly observed as the puzzling kilohertz quasi-periodic oscillations
(QPOs) in the X-ray emission from a few accreting galactic black holes and
several neutron stars.
