  We consider a scalar field interaction with a cosmic string configuration in
a flat 3-brane. The origin of this field is given by a compactification
mechanism in the context of a 5-dimensional Brans-Dicke theory. We use the
Cosmic Microwave Background Radiation constraint to analyse a possibility of
optical activity effect in connection with the Brans-Dicke parameter w and
obtain relations between the compactification modes. We show that the dilatons
produced by a cosmic string can decay into gauge bosons with masses given by
the compactification modes. The Brans- Dicke parameter w imposes stringent
constraints on the mass of the dilaton which lives in the brane and help us to
understand the energy scales. In this scenario we estimate the lifetime of the
dilaton which decays into light gauge bosons and the dependence of this
phenomenon with the Brans-Dicke parameter.
