Magnetic backgrounds and tachyonic instabilities in closed string theory

We discuss some aspects of closed superstrings in Melvin-type magnetic backgrounds. A 2-parameter family of such NS-NS backgrounds are exactly solvable as weakly coupled string models with the spectrum containing tachyonic modes. Magnetic field allows one to interpolate between free superstring theories with periodic and antiperiodic boundary conditions for the space-time fermions around some compact direction, and, in particular, between type 0 and type II string theories. Using ``9-11'' flip, this interpolation can be extended to M-theory and may be used to study the issue of tachyon in type 0 string theory. We review, following hep-th/0104238, related duality proposals, and, in particular, consider a description of type 0 theory in terms of M-theory in a curved magnetic flux background in which the type 0 tachyon appears to correspond to a state in $d=11$ supergravity fluctuation spectrum.