Frequency and width crossing of two interacting resonances in a microwave cavity

Frequency and width crossing have been observed for two coupled resonances in a microwave cavity. The cavity consisted of two nearly identical rectangular boxes. The boxes were coupled by a slit of variable width. The data are well described by a non-Hermitean S matrix leading to a $2\times 2$ non-Hermitean effective Hamiltonian. The values of the interaction strength $|v|$ for which width crossing and frequency crossing, respectively, were observed, are above and below a critical value $|v_c|$ for which one expects a joint frequency and width crossing.