In this paper we present results of (linear-dichroic) absorbance-detected magnetic resonance experiments on the FMO-complex of Prosthecochloris aestuarii at 1.2 K. We have used these results as additional criteria for testing exciton simulations of the optical spectra of this complex. We find that the photo-excited triplet state is localized on a single bacteriochlorophyll a chromophore of the complex associated with a S-1<--S-0 transition at similar to 825 nm in the Q(y)-absorption region (790-830 nm). The orientations of several optical transition moments in the Q(y)-absorption band relative to the triplet x-and y-axes of the triplet-carrying molecule have been determined. Comparison of the triplet-minus-singlet absorbance-difference spectra with simulations using parameter sets from the literature shows significant discrepancies, which can be traced to an overestimation of the dipolar interactions between the individual bacteriochlorophyll a molecules. Exciton simulations based on considerably lower dipolar interactions, presented in an accompanying paper,:show considerable improvement of the fits to the relevant optical steady-state spectra.