  We study zero-temperature Hubbard chains with periodically modulated hopping
at arbitrary filling n and magnetization m. We show that the magnetization
curves have plateaux at certain values of m which depend on the periodicity p
and the filling. At commensurate filling n a charge gap opens and then
magnetization plateaux correspond to fully gapped situations. However, plateaux
also arise in the magnetization curves at fixed n between the commensurate
values and then the plateau-value of of m depends continuously on n and can
thus also become irrational. In particular for the case of dimerized hopping
(p=2) and fixed doping we find that a plateau appears at m=1-n. In this case,
there is still a gapless mode on the plateau leading to thermodynamic behavior
which is different from a completely gapped situation.
