  The spin and charge excitations of the t--J ladder are studied by exact
diagonalization techniques for several electron densities. The various modes
are classified according to their spin (singlet or triplet excitations) and
their parity under a reflection with respect to the symmetry axis along the
chains and a finite size scaling of the related gaps is performed. At low
doping, formation of hole pairs leads to a spin gap for all $J/t$ ratios. This
phase is characterized by (at least) one vanishing energy mode {\it only} in
the charge bonding channel when $K_x\rightarrow 0$ consistent with the
existence of superconducting pairing correlations. At larger doping the spin
gap disappears. Although the anti-bonding modes remain gapped, low energy
$K_x\sim 0$ and $K_x\sim 2k_F$ spin and charge bonding modes are found
consistent with a single band Luttinger scenario. At sufficient low electron
density and above a critical value of J/t we also expect another phase of
electron pairs with gapped spin excitations.

Condensed Matter