  Recent work has shown that a temperature dependence of the shear viscosity to
entropy ratio, $\eta/s$, influences the collective flow pattern in heavy-ion
collisions in characteristic ways that can be measured by studying hadron
transverse momentum spectra and their anisotropies. Here we point out that it
also affects the pair momentum dependence of the Hanbury-Brown$-$Twiss (HBT)
radii (the source size parameters extracted from two-particle intensity
interferometry) and the variance of their event-by-event fluctuations. This
observation establishes interferometric signatures as useful observables to
complement the constraining power of single-particle spectra on the temperature
dependence of $\eta/s$.
