  We show that lightly doped YBa$_2$Cu$_3$O$_{6+y}$ has multiple holon pockets
with different areas, which lead to multiple frequencies of magnetic quantum
oscillations. Using neutron scattering data on incommensurate spin ordering we
determine these areas, which yields frequencies in good agreement with
experiments. Divergence of the effective mass observed in magnetic quantum
oscillations indicates a quantum phase transition at the oxygen content
$y\approx 0.48$. We argue that the transition is the onset of quasistatic
incommensurate magnetic order predicted by theory and observed in neutron
scattering.
