  We study the ground-state properties of the double-chain Hubbard model
coupled with ferromagnetic exchange interaction by using the weak-coupling
theory, density-matrix renormalization group technique, and Lanczos
exact-diagonalization method. We determine the ground-state phase diagram in
the parameter space of the ferromagnetic exchange interaction and band filling.
We find that, in high electron density regime, the spin gap opens and the
spin-singlet $d_{xy}$-wave-like pairing correlation is most dominant, whereas
in low electron density regime, the fully-polarized ferromagnetic state is
stabilized where the spin-triplet $p_{y}$-wave-like pairing correlation is most
dominant.
