# Effective theories of two T-dual theories are also T-dual

## TL;DR

This paper explores the relationship between T-duality and boundary conditions in open bosonic strings, demonstrating that T-dual theories are also T-dual at the effective level, with boundary conditions transforming as expected.

## Contribution

It provides a detailed analysis of how T-duality affects boundary conditions and effective theories of open strings, confirming their T-duality at the effective theory level.

## Key findings

- T-dual boundary conditions are opposite to original boundary conditions.
- Applying Dirac's procedure to boundary constraints yields $\sigma$-dependent constraints.
- Effective closed string theories derived are also T-dual.

## Abstract

We investigate how T-duality and solving the boundary conditions of the open bosonic string are related. We start by considering the T-dualization of the open string moving in the constant background. We take that the coordinates of the initial theory satisfy either Neumann or Dirichlet boundary conditions. It follows that the coordinates of T-dual theory satisfy exactly the opposite set of boundary conditions. We treat the boundary conditions of both theories as constraints, and apply the Dirac procedure to them, which results in forming $\sigma$-dependent constraints. We solve these constraints and obtain the effective theories for the solution. We show that the effective closed string theories are also T-dual.