  In this paper we address the question of coexistence of superconductivity and
ferromagnetism in the high temperature superconductor
RuSr$_2$GdCu$_2$O$_{8-\delta}$. Using a field theoretical approach we study a
one-fermion effective model of a ferromagnetic superconductor in which the
quasiparticles responsible for the ferromagnetism form the Cooper pairs as
well. We discuss the physical features which are different in this model and
the standard BCS model and consider their experimental consequences.
