Schematic picture of the lensing geometry as used in Sect. 5.4. B′ denotes the retardation corrected position of B (cf. Klioner & Kopeikin 1992; Kopeikin & Schäfer 1999). In principle there is a second photon path towards the observer (below B′). However, for the double pulsar, this signal is not only significantly weaker, but the path also comes so close to pulsar B that the photons are absorbed by the plasma-filled magnetosphere of B (cf. Lai & Rafikov 2005; Rafikov & Lai 2006b).