  We demonstrate that the kernel of the Lippmann-Schwinger equation, associated
with interactions consisting of a sum of the Coulomb plus a short range nuclear
potential, below threshold becomes degenerate. Taking advantage of this fact,
we present a simple method of calculating the effective range function for
negative energies. This may be useful in practice since the effective range
expansion extrapolated to threshold allows to extract low-energy scattering
parameters: the Coulomb-modified scattering length and the effective range.
