  We study admissible boundary conditions for a charged quantum particle in a
two-dimensional region subjected to an external magnetic field, i.e. a quantum
magnetic billiard. After reviewing some physically interesting classes of
admissible boundary conditions (magnetic Robin and chiral boundary conditions),
we turn our attention to the role of gauge transformations in a magnetic
billiard: in particular, we introduce gauge covariant boundary conditions, and
find a sufficient condition for gauge covariance which is satisfied by all the
aforementioned examples.
