Metric and coupling reversal in string theory

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