Scientific articles on tandems

Scientific articles on learning a foreign language in tandems:

1) https://www.tandfonline.com/doi/abs/10.1080/01434632.2017.1315948

2) https://journals.openedition.org/apliut/7842

3) https://www.academia.edu/38852828/Goals_and_goal_setting_in_tandem_language_learning 

4) https://www.tandfonline.com/doi/abs/10.1080/13670050.2018.1516188