  In the two-leg S=1/2 ladders of Sr_14Cu_24O_41 a modulation of the exchange
coupling arises from the charge order within the other structural element, the
CuO_2 chains. In general, breaking translational invariance by modulation
causes gaps within the dispersion of elementary excitations. We show that the
gap induced by the charge order can drastically change the magnetic Raman
spectrum leading to the sharp peak observed in Sr_14Cu_24O_41. This sharp Raman
line gives insight in the charge-order periodicity and hence in the
distribution of carriers. The much broader spectrum of La_6Ca_8Cu_24O$_41
reflects the response of an undoped ladder in the absence of charge order.
