Metric and coupling reversal in string theory Original Research Article

Invariance under reversing the sign of the metric image and/or the sign of the string coupling field image, where image, leads to four possible universes denoted 1, image, image, image according as image, image, image, image, respectively. Universe 1 is described by conventional string/M theory and contains all M, D, F and NS branes. Universe image contains only image, D3 and D7. Universe image contains only D1, D5, D9 and Type I. Universe image contains only F1 and NS5 of IIB and heterotic SO(32).