  We consider a 2-parameter class of solvable closed superstring models which
`interpolate' between Kaluza-Klein and dilatonic Melvin magnetic flux tube
backgrounds. The spectrum of string states has similarities with Landau
spectrum for a charged particle in a uniform magnetic field. The presence of
spin-dependent `gyromagnetic' interaction implies breaking of supersymmetry and
possible existence (for certain values of magnetic parameters) of tachyonic
instabilities. We study in detail the simplest example of the Kaluza-Klein
Melvin model describing a superstring moving in flat but non-trivial 10-d space
containing a 3-d factor which is a `twisted' product of a 2-plane and an
internal circle. We also discuss the compact version of this model constructed
by `twisting' the product of the two groups in SU(2) x U(1) WZNW theory without
changing the local geometry (and thus the central charge). We explain how the
supersymmetry is broken by continuous `magnetic' twist parameters and comment
on possible implications for internal space compactification models.
(Contribution to the Proceedings of the 1995 Erice School "String Gravity and
Physics at the Planck Scale")
