  We argue that the low- frequency quantum oscillations observed recently in
the vortex state of underdoped ortho II-YBCO have the same origin as in other
strongly correlated electronic systems. Superconductivity driven by strong
interactions on several Fermi surfaces creates proximity gaps on the other. The
gap transferred on a small-sized pocket from larger Fermi surfaces is small. In
case of unconventional pairing symmetry the induced gap is proportional to
ratio of the oscillations' frequencies for small and large Fermi surfaces. The
gap is in inverse proportion to mass enhancement on the latter. We share the
view that small pockets themselves are certain feature of topology of the band
Fermi surfaces.
