  This paper investigates the use of gravitational time delays of macro-lenses
to constrain a possible photon mass. The time delay between the 2 compact
images of a source of massive photons is computed. Explicit expressions are
given for Schwarzschild and singular isothermal lenses. In the latter case, the
time delay is very insensitive to the photon mass. Modeling lens galaxies by a
singular isothermal model and a central supermassive black hole, the photon
mass-dependent part of the time delay between the compact images is shown to be
proportional to the mass of the black hole. The sensitivity of time delays to
the photon mass is illustrated by a bound obtained from 3 AGN which have
measurements in several passbands. The bound obtained is comparable to the
limit with the deflection of radio waves by the Sun.
