HETEROTIC STRINGS IN A UNIFORM MAGNETIC-FIELD

An exact conformal model representing a constant magnetic field backgr ound in heterotic string theory is explicitly solved in terms of free creation/annihilation operators. The spectrum of physical states is ex amined for different possible embeddings of the magnetic U(1) subgroup . We find that an arbitrarily small magnetic field gives rise to an in finite number of tachyonic excitations corresponding to charged vector states of the massless level and to higher level states with large sp ins and charges.