  We derive the delay in travel time of photons due to the spin of a body both
inside a rotating shell and outside a rotating body. We then show that this
time delay by the spin of an astrophysical object might be detected in
different images of the same source by gravitational lensing; it might be
relevant in the determination of the Hubble constant using accurate
measurements of the time delay between the images of some gravitational lens
systems. The measurement of the spin-time-delay might also provide a further
observable to estimate the dark matter content in galaxies, clusters, or
super-clusters of galaxies.
