  The compounds A$_2$Cu$_3$O(SO$_4$)$_3$ (A=Na, K) are characterized by copper
hexamers which are weakly coupled to realize antiferromagnetic order below TN=3
K. They constitute novel quantum spin systems with S=1 triplet ground-states.
We investigated the energy-level splittings of the copper hexamers by inelastic
neutron scattering experiments covering the entire range of the magnetic
excitation spectra. The observed transitions are governed by very unusual
selection rules which we ascribe to the underlying spin-coupling topology. This
is rationalized by model calculations which allow an unambiguous interpretation
of the magnetic excitations concerning both the peak assignments and the nature
of the spin-coupling parameters.
