  We investigate a double layer system with tight-binding hopping, intra-layer
and inter-layer interactions, as well as a Josephson like coupling. We find
that an antiferromagnetic spin polarization induces additional spin-triplet
pairing (with $S_z =0$) to the singlet order parameter. This causes an undamped
collective mode in the superconducting state below the particle-hole threshold,
which is interpreted as a Goldstone excitation.
