  Motivated by magnetization studies of the "telephon number compound"
Sr_14Cu_24O_41 we investigate doped Heisenberg spin rings by means of complete
numerical diagonalization of a Heisenberg Hamiltonian that depends
parametrically on hole positions. A comparison with experimental magnetization
data reveals rather accurate information about the screened electrostatic
interaction between the charged holes on the ring which appears to be
astonishingly strong.
