Abstract: A new isomorphism invariant of certain measure preserving flows, using sequences of integers, is introduced. Using this invariant, we construct families of type III_0 systems which are not orbit equivalent. In particular we construct an uncountable family of nonsingular ergodic transformations, each having an associated flow that is approximately transitive (and therefore of zero entropy), with the property that the transformations are pairwise not orbit equivalent. This is joint work with A. Dooley and D. Ralston.