The influence of world-sheet boundary condensates on the toroidal
compactification of bosonic string theories is considered. At the special
points in the moduli space at which the closed-string theory possesses an
enhanced unbroken G×G symmetry (where G is a semi-simple product of
simply laced groups) a scalar boundary condensate parameterizes the coset
G×G/G. Fluctuations around this background define an open-string
generalization of the corresponding chiral nonlinear sigma model. Tree-level
scattering amplitudes of on-shell massless states (\lq pions') reduce to the
amplitudes of the principal chiral model for the group G in the low energy
limit. Furthermore, the condition for the vanishing of the renormalization
group beta function at one loop results in the familiar equation of motion for
that model. The quantum corrections to the open-string theory generate a mixing
of open and closed strings so that the coset-space pions mix with the
closed-string G×G gauge fields, resulting in a Higgs-like breakdown of
the symmetry to the diagonal G group. The case of non-oriented strings is
also discussed.Comment: 32 pages, LaTeX, 2 figures in uuencoded fil