  Standard interpolating operators for charged mesons, e.g. $J_{B} = \bar b i
\gamma_5 u$ for $B^-$, are not gauge invariant in QED and therefore problematic
for perturbative methods. We propose a gauge invariant interpolating operator
by adding an auxiliary charged scalar $\Phi_B$, ${\cal J}_{B}^{(0)} = J_B \,
\Phi_B$, which reproduces all the universal soft and collinear logs. The
modified LSZ-factor is shown to be infrared finite which is a necessary
condition for validating the approach. At ${\cal O}(\alpha)$, this is
equivalent to a specific Dirac dressing of charged operators. A generalisation
thereof, using iterated integrals, establishes the equivalence to all orders
and provides a transparent alternative viewpoint. The method is discussed by
the example of the leptonic decay $B^- \to \ell^- \bar \nu$ for which a
numerical study is to follow. The formalism itself is valid for any spin,
flavour and set of final states (e.g. $B^- \to \pi^0 \ell^- \bar \nu$).
