  We present an exploratory study of leading isospin breaking effects in
mesonic masses using $O(a)$ improved Wilson fermions with open boundaries.
Isospin symmetry is explicitly broken by distinct masses and electric charges
of the up and down quarks. In order to be able to make use of existing
isosymmetric QCD gauge ensembles we apply reweighting techniques. The path
integral describing QCD+QED is expanded perturbatively in powers of deviations
in the quark masses and the inverse strong coupling as well as the
electromagnetic coupling. We have constructed QED$_{\mathrm{L}}$, which we use
as a finite volume formulation of QED, for open boundaries. We will also give a
first insight into contributions from quark disconnected diagrams.
