  We consider oscillator ensembles consisting of subpopulations of identical
units, with a general heterogeneous coupling between subpopulations. Using the
Watanabe-Strogatz ansatz we reduce the dynamics of the ensemble to a relatively
small number of dynamical variables plus constants of motion. This reduction is
independent of the sizes of subpopulations and remains valid in the
thermodynamic limits. The theory is applied to the standard Kuramoto model and
to the description of two interacting subpopulations, where we report a novel,
quasiperiodic chimera state.
