We construct a two-parameter family of 2-particle Hamiltonians closed under
the duality operation of interchanging the (relative) momentum and coordin
ate. Both coordinate and momentum dependence are elliptic, and the modulus
of the momentum torus is a non-trivial function of the coordinate. This mod
el contains as limiting cases the standard Ruijsenaars-Calogero and Toda fa
mily of Hamiltonians, which are at most elliptic in the coordinates, but no
t in the momenta. (C) 2000 Elsevier Science B.V. All rights reserved.