Abstract
We consider open strings in an external constant magnetic field H. For an (infinite) sequence of critical values of H an increasing number of (highest spin component) states lying on the first Regge trajectory becomes tachyonic. In the limit of infinite H all these states are tachyons (with a common tachyonic mass) both in the case of the bosonic string and for the Neveu-Schwarz sector of the fermionic string. This result generalizes to extended object the same instability which occurs in ordinary non-Abelian gauge theories. The Ramond states always have positive square masses as is the case for ordinary QED. The weak field limit of the mass spectrum is the same as for a field theory with gyromagnetic ratio gS=2 for all charged spin states. This behavior suggests a phase transition of the string as it has been argued for the ordinary electroweak theory.
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