  The Fourier transform of the observed magnetic quantum oscillations (MQO) in
YBa$_{2}$Cu$_{3}$O$_{6+\delta }$ high-temperature superconductors has a
prominent low-frequency peak with two smaller neighbouring peaks. The
separation and positions of these three peaks are almost independent of doping.
This pattern has been explained previously by rather special, exquisitely
detailed, Fermi-surface reconstruction. We propose that these MQO have a
different origin, and their frequencies are related to the bilayer and
inter-bilayer electron hopping rather than directly to the areas of tiny
Fermi-surface pockets. Such so-called "slow oscillations" explain more
naturally many features of the observed oscillations and allow us to estimate
the inter-layer transfer integrals and in-plane Fermi momentum.
