  Sine-square deformation, a recently found modulation of the coupling strength
in certain statistical models, is discussed in the context of two-dimensional
conformal field theories, with particular attention to open/closed string
duality. This deformation is shown to be non-trivial and leads to a divergence
in the worldsheet metric. The structure of the vacua of the deformed theory is
also investigated. The approach advocated here may provide an understanding of
string duality through the worldsheet dynamics.
