§ 38 (5) Law on Higher Education of the Land of Baden-Wuerttemberg (LHG):

Persons who are working on her or his doctoral thesis and have been accepted as a doctoral student since 30.03.2018 have to be enrolled as doctoral students. As soon as you have been accepted as a doctoral candidate, you are obliged to enrol in the semester in which you were accepted.

Doctoral students who were accepted before 30.03.3018 can enrol but are not obliged to do so.

Doctoral students* who are full-time students at the University of Ulm can be exempted from the obligation to enrol if they make a declaration to the president that they do not want to be enrolled.