We study the occurrence of plateaux and jumps in the magnetization curves of
a class of frustrated ladders for which the Hamiltonian can be written in terms
of the total spin of a rung. We argue on the basis of exact diagonalization of
finite clusters that the ground state energy as a function of magnetization can
be obtained as the minimum - with Maxwell constructions if necessary - of the
energies of a small set of spin chains with mixed spins. This allows us to
predict with very elementary methods the existence of plateaux and jumps in the
magnetization curves in a large parameter range, and to provide very accurate
estimates of these magnetization curves from exact or DMRG results for the
relevant spin chains.Comment: 14 pages REVTeX, 7 PostScript figures included using psfig.sty; this
is the final version to appear in Eur. Phys. J B; some references added and a
few other minor change
