What is the effective mass of an exciton?

In the effective-mass approximation, the mass M* of an exciton is just the sum of the electron and hole masses, me*+ mh*. However, the effective-mass approximation is invalid if the forces are strong. Here we derive a plausible formula for the n th bound state of the exciton: Mn*= (me* + mh*) / (1 - Kn/W), where Kn is the kinetic energy in the bound state and W is one-half the sum of the electron and hole bandwidths. For Wannier excitons, Kn/W << 1, while for Frenkel excitons, K1s/W-- 1, and the composite particle is effectively localized.