Highest resolution Event Horizon Telescope (EHT) observations will

probably soon tell us more about the supermassive black hole at

the Galactic Centre (Sgr A*) and the cores of active galactic nuclei

(AGN). It might also help to clarify the long-standing question

whether the central massive objects in AGN are instead close pairs

of black holes. Mergers of supermassive black hole pairs would provide

the strongest gravitational wave signals. I will present examples

of how we identify potential close binary black hole candidates

based on the combined analysis of high resolution radio interferometric

(VLBI) observations and multi- wavelength data. I will also

provide an outlook on the scientific prospects with regard to future

EHT-observations.