  Models of closed superstrings in certain curved NS-NS magnetic flux
backgrounds are exactly solvable in terms of free fields. They interpolate
between free superstring theories with periodic and antiperiodic boundary
conditions for 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 provides an interesting setting
for a study of tachyon problem in closed string theory. Starting with a general
2-parameter family of such Melvin-type models, we present several new magnetic
flux backgrounds in 10-d string theory and 11-d M-theory and discuss their
tachyonic instabilities. In particular, we suggest a description of type 0B
theory in terms of M-theory in curved magnetic flux background, which supports
its conjectured SL(2,Z) symmetry, and in which the type 0 tachyon appears to
correspond to a state in 11-d supergravity multiplet. In another ``T-dual''
description, the tachyon is related to a winding membrane state.
