Time Delay in Strong Field Limit Gravitational Lensing

We calculate the time delay between different relativistic images formed by black hole gravitational lensing in the strong field limit. We show that different types of black holes are characterized by different time delays, so that one further quantity can eventually become available for the classification of black holes and a more precise determination of their parameters. The time delay for the black hole at the center of our Galaxy is few minutes, but for supermassive black holes with 10^9 solar masses in the neighbourhood of the Local Group the time delay amounts to few days, thus being measurable in principle