  The double differential dilepton spectrum $dN/ (dM_\perp^2 dM^2)$ at fixed
transverse mass $M_\perp$ allows a direct access to the vector meson spectral
functions. Within a fireball model the sensitivity of $dN/ (dM_\perp^2 dM^2)$
against variations of both the in-medium properties of mesons and the dynamics
of the fireball is investigated. In contrast to the integrated invariant-mass
spectrum $dN/dM^2$, in the spectrum $dN/dM^2 dM_\perp^2$ with fixed $M_\perp$
the $\omega$ signal is clearly seen as bump riding on the $\rho$ background
even in case of strong in-medium modifications.
