GSO projections via SPT phases

Understand

We point out that the choice of phases in GSO projections can be accounted for by a choice of fermionic SPT phases on the worldsheet of the string.

• This point of view not only easily explains why there are essentially two type II theories, but also predicts that there are unoriented type 0 theories labeled by n mod 8, and that there is an essentially unique choice of the type I worldsheet theory.
• We also discuss the relationship between this point of view and the K-theoretic classification of D-branes.