  The goals of this article are twofold : 1) to compute the conjugate locus of
a geodesic that lies in the center of a simply connected, 2-step nilpotent Lie
group with a left invariant metric 2) compare the isometry types of two such
nilpotent Lie groups whose conjugate loci for central geodesics are "the same"
in a suitable sense. The first goal is achieved. The second is elusive, but we
obtain a partial result.
