Mass spectra of dual strings

Dual strings seem to correspond uniquely to certain subgroups of the modular group. From three axioms, one of which is a tentative simplicity assumption, a finite number of possible mass spectra is derived, but not yet the corresponding amplitudes. Above space-time dimension 8, this approach yields just the mass spectra of the known models, plus one additional candidate for 10 dimensions. Further models exist probably for dimensions 8, 6, and 4, and perhaps for dimensions 5 and 3. The mass spectrum of one candidate for the “correct model” is determined. It yields only mesons of charge zero, but otherwise the agreement with experiment is amazing.