Truncations of the D9-brane action and type-I strings

TL;DR

This paper extends the understanding of D9-brane action truncations in type-I superstring theory to generic backgrounds, linking two truncation methods to different consistent ten-dimensional string theories.

Contribution

It generalizes previous flat-background results to arbitrary backgrounds and clarifies the correspondence between truncation methods and specific type-I string theories.

Findings

Two truncation methods correspond to different type-I string theories.

Extension of D9-brane action truncation to generic backgrounds.

Identification of the open sector with low-energy effective actions.

Abstract

The low-energy effective action of type-I superstring theory in ten dimensions is obtained performing a truncation of type-IIB supergravity in a background where D9-branes are present. The open sector corresponds to the first order in the low-energy expansion of the D9-brane action in a type-I background. In hep-th/9901055 it was shown that there are two ways of performing a type-I truncation of the D9-brane action, and the resulting truncated action was obtained in a flat background. We extend this result to a generic type-I background, and argue that the two different truncations are in correspondence with the open sector of the low-energy effective action of the two different consistent ten-dimensional type-I string theories, namely the SO(32) superstring and the $USp(32)$ non-supersymmetric string.