# Are there string backgrounds which can't be described by first quantized string theory?

In the strong coupling limit of type IIA and heterotic E8 string theory, we get 11 dimensional M-theory in which we have no strings. Instead, we have M2 branes.

Are there any other backgrounds in string theory which can't be described by first quantized string theory?

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We have mentioned three 10-dimensional superstring backgrounds. The remaining 10-dimensional supersymmetric string vacua are those with the $SO(32)$ gauge group - the HO heterotic theory and type I string theory - and they're S-dual to each other. The heterotic strings at the strong coupling become type I D1-branes which are heavy. The type I strings, because they're unoriented, can't carry any string-like conserved charge, become unstable, and they don't leave any "visible traces" in the heterotic string spectrum at all.