  The superfluid drag effect, in hydrodynamics of pulsating neutron stars, is
conventionally described with the aid of the entrainment matrix relating the
mass currents with the velocities of superfluid flows in the system. Equations
for the entrainment matrix of a superfluid mixture of neutrons and protons are
derived with allowance for the strong dependence of the energy gaps on the
velocities of superfluid flows. The calculations are carried out in the frame
of the Fermi-liquid theory. The equations obtained are highly nonlinear.
Numerical solutions to the equations for some typical cases demonstrate that
the components of the entrainment matrix possess a highly nonlinear dependence
on the velocities of the two superflows simultaneously. This effect, previously
ignored, can greatly influence the dynamics of neutron stars.
