ar X iv : h ep - t h / 01 09 02 1 v 2 1 9 Se p 20 01 Exceptional boundary states at c = 1 Romuald

We consider the CFT of a free boson compactified on a circle, such that the compactification radius R is an irrational multiple of Rselfdual. Apart from the standard Dirichlet and Neumann boundary states, Friedan suggested [1] that an additional 1-parameter family of boundary states exists. These states break U(1) symmetry of the theory, but still preserve conformal invariance. In this paper we give an explicit construction of these states, show that they are uniquely determined by the Cardy-Lewellen sewing constraints, and we study the spectrum in the ‘open string channel’, which is given here by a continous integral with a nonnegative measure on the space of conformal weights.