This paper presents an investigation of the seven invitatory antiphons in the Birgittine sisters’ weekly office, the Cantus sororum (CS). This is not only the first study if these chants, but also an attempt at suggesting a method for studying melodies and texts separately in order to uncover how they were composed. These antiphons have been analyzed using sources from ca. 1500 from the Birgittine abbey Mariënwater in the Netherlands. Just like other already studied repertoires in CS, the invitatories are a mix of borrowings, adaptations, and unica, but this study offers in-depth analyses of how this was done. By this new patterns in the compilation of CS have been discerned. The following strategies were uncovered:

• Unique texts combined with melodies resembling known melodies outside CS.
• Texts and melodies resembling variants outside CS but combined in a new way.
• Both text and melody unknown outside CS.
• From other sources known texts and melodies combined in a new way.

This analysis has allowed for a better picture of how borrowings and adaptations have been combined into new combinations where also the Treitlerian idea of pitch groups has been considered. The paper demonstrates how these four strategies were used and how new invitatories were created for the Birgittine sisters, whereas earlier research has emphasized the Birgittine’s dependence on previously existing materials.