Destruction of Fermion Zero Modes on Cosmic Strings

I examine the existence of zero-energy fermionsolutions (zero modes) on cosmic strings in an SO (10)grand unified theory. The current-carrying capability ofa cosmic string formed at one phase transition can be modified at subsequent phasetransitions. I show that the zero modes may be destroyedand the conductivity of the string altered. I discussthe cosmological implications of this, and show that it allows vorton bounds to be relaxed.