  It is expected that in the interior of compact stars a proton superconductor
coexists with and couples to a neutron superfluid. Starting from a
field-theoretical model for two complex scalar fields - one of which is
electrically charged - we derive a Ginzburg-Landau potential which includes
entrainment between the two fluids and temperature effects from thermal
excitations of the two scalar fields and the gauge field. The Ginzburg-Landau
description is then used for an analysis of the phase structure in the presence
of an external magnetic field. In particular, we study the effect of the
superfluid on the flux tube phase by computing the various critical magnetic
fields and deriving an approximation for the flux tube interaction. As a
result, we point out differences to the naive expectations from an isolated
superconductor, for instance the existence of a first-order flux tube onset,
resulting in a more complicated phase structure in the region between type-I
and type-II superconductivity.
