Magnetic backgrounds and tachyonic instabilities in closed superstring theory and M-theory

Models of closed superstrings in certain curved NSNS magnetic flux backgrou nds are exactly solvable in terms of free fields. They interpolate between free superstring theories with periodic and antiperiodic boundary condition s for fermions around some compact direction, and, in particular, between t ype 0 and type II string theories. Using "9-11" flip, this interpolation ca n be extended to M-theory and provides an interesting setting for study of tachyon problem in closed string theory. Starting with a general 2-paramete r family of such Melvin-type models, we present several new magnetic flux b ackgrounds in 10-d string theory and 11-d M-theory and discuss their tachyo nic instabilities. In particular, we suggest a description of type OB theor y in terms of M-theory in curved magnetic flux background, which supports i ts conjectured SL(2, Z) symmetry, and in which the type 0 tachyon appears t o correspond to a state in d = 11 supergravity multiplet. In another "T-dua l" description, the tachyon is related to a winding membrane state. (C) 200 1 Published by Elsevier Science B.V.