the framework of QCD sum rules [@SVZ] failed to produce acceptable results. A possible reason for this is that the background field technique developed in [@Ioffe:1983ju] (see also [@Balitsky:1983xk] for an equivalent approach) is only applicable for the case that the participating initial state and final state hadrons have equal masses. Technically, this restriction arises because the contribution of interest can only be isolated as the double-pole contribution in the hadron momentum. This is the case for e.g. the calculation of nucleon magnetic moments which was the primary task of [@Ioffe:1983ju; @Balitsky:1983xk], but it is not a good approximation for the $N \to \Delta\gamma$ radiative transition.

The problem of calculating the transitions between hadrons of different mass is known for a long time and provided the main motivation for the development of an alternative approach [@Balitsky:1989ry; @Chernyak:1990ag], now known as light-cone sum rules (LCSRs). In this technique, an infinite series of the “induced condensates” (in the language of [@Ioffe:1983ju]) is resummed in a function that has the physical meaning of a photon distribution amplitude and describes the probability amplitude to find a quark and an antiquark in the real photon, with given momentum fractions and at small transverse separations. The 