On M-Theory and the Symmetries of Type II String Effective Actions

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We study the ``ordinary'' Scherk-Schwarz dimensional reduction of the bosonic sector of the low energy effective action of a hypothetical M-theory on S¹ × S¹ T². We thus obtain the low energy effective actions of type IIA string theory in both ten and nine space-time dimensions. We point out how to obtain the O(1, 1) invariance of the NS-NS sector of the string effective action correctly in nine dimensions. We dimensionally reduce the type IIB string effective action on S¹ and show that the resulting nine dimensional theory can be mapped, purely from the bosonic consideration, exactly to the type IIA theory by an O(1, 1) or Buscher's T-duality transformations. We then give a dynamical argument, in analogy with that for the type IIB theory in ten dimensions, to show how an S-duality in the type IIA theory can be understood from the underlying nine dimensional theory by compactifying M-theory on a T-dual torus T².