  Series expansions at T=0 are used to study properties of the half-filled
$t-J$ ladder doped with one or two holes, and at quarter filling. Dispersion
curves are obtained for one-hole symmetric and antisymmetric (bonding and
antibonding) excitations and for the two-hole bound state. The line in the
phase diagram which separates bound and unbound states is determined. For
quarter filling we compute the ground state energy and estimate the location of
the phase separation line. Comparisons with other numerical and analytical
results are presented.
