  We analyze a non-symmetric coupling of interior penalty discontinuous
Galerkin and boundary element methods in two and three dimensions. Main results
are discrete coercivity of the method, and thus unique solvability, and
quasi-optimal convergence. The proof of coercivity is based on a localized
variant of the variational technique from [F.-J. Sayas, The validity of
Johnson-N\'edel\'ec's BEM-FEM coupling on polygonal interfaces, {\em SIAM J.
Numer. Anal.}, 47(5):3451--3463, 2009]. This localization gives rise to terms
which are carefully analyzed in fractional order Sobolev spaces, and by using
scaling arguments for rigid transformations. Numerical evidence of the proven
convergence properties has been published previously.
