  The spectra of dileptons radiated from the fireballs formed in high-energy
heavy-ion collisions have been successfully used to investigate key properties
of hot and dense QCD matter. In this paper we study polarization observables
which have thus far received little attention. Microscopic calculations of
in-medium electromagnetic spectral functions have thus far mostly focused on
integrated yields which are proportional to the sum of the longitudinal and
transverse components of the virtual photon's selfenergy. Photon polarization
results from the difference of these components, which in general does not
vanish for lepton pairs at finite three-momentum relative to the heat bath (and
is maximal for fully transverse real photons). Using a model that successfully
describes dilepton spectra in heavy-ion collisions, with hadronic emission via
medium-modified vector mesons and quark-antiquark annihilation constrained by
lattice QCD, we compute polarization observables in different dilepton mass
bins and confront them with data of the HADES and NA60 experiments.
