Metric and coupling reversal in string theory
  Invariance under reversing the sign of the metric G_{MN}(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) goes to (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). 
M.J. Duff and Jussi Kalkkinen