  We study the electron-loss-to-continuum (ELC) cusp experimentally and
theoretically by comparing the ionization of U$^{89+}$ projectiles in
collisions with N$_2$ and Xe targets, at a beam energy of 75.91 MeV/u. The
coincidence measurement between the singly ionized projectile and the energy of
the emitted electron is used to compare the shape of the ELC cusp at weak and
strong perturbations. A significant energy shift for the centroid of the
electron cusp is observed for the heavy target of Xe as compared to the light
target of N$_2$. Our results provide a stringent test for fully relativistic
calculations of double-differential cross sections performed in the first-order
approximation and in the continuum-distorted-wave approach.
