BOUNDARY EFFECTS IN STRING THEORY AND THE EFFECTIVE STRING COUPLING.

The effect of the ideal boundaries of infinite-genus surfaces and Dirichlet boundaries on the value of the string coupling is evaluated. It is demonstrated that the value coincides with the unified gauge coupling and it is not altered by the inclusion of curved four-dimensional spacetimes in the path integral for quantum gravity or boundaries with fractional dimension and nonzero harmonic measure. [ABSTRACT FROM AUTHOR]