Metric and coupling reversal in string theory

Abstract Invariance under reversing the sign of the metric G M N ( x ) and/or the sign of the string coupling field H ( x ) , where 〈 H ( x ) 〉 = g s , 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).