  In this theoretical investigation we predict an unusual interaction between a
discrete state and a continuum of states, which is closely related to the case
of Fano-interference. It occurs in a GaAs/AlxGa1-xAs quantum well between the
lowest light-hole exciton and the continuum of the second heavy-hole exciton.
Unlike the typical case for Fano-resonance, the discrete state here is outside
the continuum; we use uniaxial stress to tune its position with respect to the
onset of the continuum. State-of-the art calculations of absorption spectra
show that as the discrete state approaches the continuum, a doublet structure
forms which reveals anticrossing behaviour. The minimum separation energy of
the anticrossing depends characteristically on the well width and is unusually
large for narrow wells. This offers striking evidence for the strong underlying
valence-band mixing. Moreover, it proves that previous explanations of similar
doublets in experimental data, employing simple two-state models, are
incomplete.
