  The phase diagram of a frustrated S=1/2 antiferromagnetic spin ladder with
additional next-nearest neighbor exchanges, both diagonal and inchain, is
studied by a weak-coupling effective field theory approach combined with exact
diagonalization for finite systems. In addition to two known phases with
rung-singlet and Haldane-type ground states, we observe two new phases with
dimerization along the chains. Furthermore, the transitions between the
different phases are studied and shown to be either first order or to belong to
the universality class of the two-dimensional Ising model. The nature of
elementary excitations is discussed briefly.
