We consider an extension of the voter model in which a set of interacting elements (agents) can be in either of two equivalent states (A or B) or in a third additional mixed (AB) state. The model is motivated by studies of language competition dynamics, where the AB state is associated with bilingualism. We study the ordering process and associated interface and coarsening dynamics in regular lattices and small world networks. Agents in the AB state define the interfaces, changing the interfacial noise driven coarsening of the voter model to curvature driven coarsening. We argue that this change in the coarsening mechanism is generic for perturbations of the voter model dynamics. When interaction is through a small world network the AB agents restore coarsening, eliminating the metastable states of the voter model. The time to reach the absorbing state scales with system size as τ ~ ln N to be compared with the scaling as τ ~ N for the voter model in a small world network.

We acknowledge financial support form the MEC (Spain) through project CONOCE2 (FIS2004-00953). XC also acknowledges financial support from a PhD fellowship of the Govern de les Illes Balears (Spain).

16 pages, 6 figures.-- ArXiv pre-print: http://arxiv.org/abs/physics/0609079.