  We present an exploratory study of leading isospin breaking effects in
mesonic masses using $O(a)$ improved Wilson fermions. 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 the light quarks' mass deviations and
the electromagnetic coupling. We employ QED$_{\mathrm{L}}$ as a finite volume
formulation of QED.
