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A classification of ten-dimensional closed-fermionic-string models with world-sheet superconformal invariance, modular invariance, and physical states with proper space-time statistics is presented. This classification is based on an analysis of the spin structures of the Neveu-Schwarz-Ramond fermions and the fermionic variables used in representing the internal-symmetry space.
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Kawai et al. (1986) studied this question.
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