  The magnetic oscillations in YBCO high-temperature superconductors have been
widely studied over the last decade and consist of three equidistant low
frequencies with a central frequency several times more intense than its two
shoulders. This remains a puzzle in spite of numerous attempts to explain the
corresponding small Fermi-surface pockets. Furthermore the ARPES data indicate
only four Fermi-arcs with bilayer splitting, and show no sign of such small
areas in the Fermi surface. Here we argue that the magnetic oscillations
measured in under-doped bilayer high temperature superconductors, in particular
YBa$_{2}$Cu$_{3}$O$_{6+\delta }$, provide a measure of the interplanar
electronic coupling rather than the areas of fine-grain reconstruction of the
Fermi surfaces coming from induced charge density waves. This identification is
based on the relative intensities of the different peaks, as well as their
angular dependence, which points to an effective Fermi surface that is larger
than the oscillation frequencies, and is compatible with several indications
from ARPES. The dominance of such frequencies with respect to the fundamental
frequencies from the Fermi surface is natural for a strongly correlated
quasi-two dimensional electronic systems where non-linear mixings of
frequencies are more resistant to sample inhomogeneity.
