  In quantum systems with short-range interactions, causality imposes
nontrivial constraints on low-energy scattering parameters. We investigate
these causality constraints for systems where a long-range Coulomb potential is
present in addition to a short-range interaction. The main result is an upper
bound for the Coulomb-modified effective range parameter. We discuss the
implications of this bound to the effective field theory for nuclear halo
systems. In particular, we consider several examples of proton--nucleus and
nucleus--nucleus scattering. For the bound-state regime, we find relations for
the asymptotic normalization coefficients (ANCs) of nuclear halo states. As an
application of these relations, we extract the ANCs of the excited 2+ and 1-
states in O-16 from alpha--C-12 scattering data.
