Internships at the ESA/EAC are currently being offered, giving Master's students the opportunity to spend 3-6 months at the EAC in Cologne. These could also be used for joint Master's theses.
The current calls for applications involve activities in the Spaceship EAC and specific work at the LUNA Analogue Facility. They can be found here on the ESA website:
ESA - Launch your career in space! Apply now to the ESA Student Internship Programme