  We study time-changes of unipotent flows on finite volume quotients of
semisimple linear groups, generalising previous work by Ratner on time-changes
of horocycle flows. Any measurable isomorphism between time-changes of
unipotent flows gives rise to a non-trivial joining supported on its graph.
Under a spectral gap assumption on the groups, we show the following rigidity
result: either the only limit point of this graph joining under the action of a
one-parameter renormalising subgroup is the trivial joining, or the isomorphism
is "affine", namely it is obtained composing an algebraic isomorphism with a
(non-constant) translation along the centraliser.
