COMPACTIFICATIONS OF F-THEORY ON CALABI-YAU THREEFOLDS AT CONSTANT COUPLING

Generalizing the work of Sen, we analyze special points in the moduli space of the compactification of the F-theory on elliptically fibered Calabi-Yau threefolds where the coupling remains constant. These conta in points where they can be realized as orbifolds of six torus T-6 by Z(m) x Z(n)(m,n = 2,3,4,6). At various types of intersection points of singularities, we find that the enhancement of gauge symmetries arise s from the intersection of two kinds of singularities. We also argue t hat when we take the Hirzebruch surface as a base for the Calabi-Yau t hreefold, the condition for constant coupling corresponds to the case where the point Wie instantons coalesce, giving rise to enhanced gauge group of Sp(k). (C) 1998 Published by Elsevier Science B.V.