Self-Dual Point and Unification

We hypothesize that the vacuum is spontaneously shifted from the self-dualpoint in the moduli space, at which all dimensions are compact, and that thetrue vacuum results from the decompactification of the four space-timedimensions and remains in the vicinity of the self-dual point for extradimensions. On the basis of our hypotheses we explore new types of the couplingunification which are not realized in the four-dimensional effective theory butin the framework of the higher dimensional theory. In the $SU(6) \timesSU(2)_R$ string model, if the SU(6) gauge group lives in the world-volume of9-branes and the $SU(2)_R$ in the world-volume of 5-branes, or {\it viceversa}, unification solutions can be found in the vicinity of the self-dualpoint.Comment: 12 pages, latex, no figures, final version to appear in Prog. Theor. Phys. Vol. 102 (1999) No.