A dimer is considered where the two monomers are coupled by dipole-dipole interactions. Upon electronic excitation the interaction leads to a drastic change of the dimer absorption spectrum compared to that of the non-interacting monomers. The method presented here uses sum rules, to obtain information on the interaction strength between the monomers and the conformation of the dimer directly from the measured spectra. Comparison with previous results [A. Eisfeld, L. Braun, W.T. Strunz, J.S. Briggs, J. Beck, V. Engel, J. Chem. Phys. 122 (2005) 134103], obtained by diagonalisation of the dimer Hamiltonian, shows remarkable agreement. (C) 2007 Elsevier B.V. All rights reserved.