An unoriented theory containing open and closed superstrings. Consistent gauge and gravitational anomaly cancellation yields the familiar gauge algebra. Open-string Chan-Paton factors supply the gauge degrees of freedom, while closed strings contain gravity. It is related by strong/weak duality to the corresponding heterotic theory.
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Type I string theory is one of the five consistent superstring theories in ten dimensions. It is part of the broader framework of string theory, which posits that the fundamental constituents of the universe are one-dimensional "strings" rather than point-like particles.