  We study the structure of two-body s-wave bound states as well as resonances
in the threshold energy region. We focus on the single-channel scattering where
the scattering length and the effective range are given by real numbers. It is
shown that, in the energy region where the effective range expansion is valid,
the properties of resonances are constrained only by the position of the pole.
We find that the compositeness defined through the analytic continuation of the
field renormalization constant is pure imaginary and normalized for resonances.
We discuss the interpretation of this quantity by examining the structure of
the hadron resonance Lambda_c(2595) in the pi Sigma_c scattering. We show that
the Lambda_c(2595) resonance requires an unnaturally large effective range and
hence it is not likely a pi Sigma_c molecule.
