The present notes are based on three lectures, each ninety minutes long, prepared for the international school "Integrability, dualities and deformations", hosted by Universidade de Santiago de Compostela, from August 23 to August 27, 2021. These lectures, aimed at graduate students, require only a basic knowledge of string theory. The main goal is to introduce α ′ -corrections to the gravitational sector of different formulations of closed string theory and to reformulate them using novel techniques based on double field theory.