  We study plasma effects on radiative transitions (e.g., decay of excited
states of atoms or atomic nuclei) in a dense plasma at the transition
frequencies $\omega \lesssim \omega_p$ (where $\omega_p$ is the electron plasma
frequency). The decay goes through four channels -- the emission of real
transverse and longitudinal plasmons as well as the emission of virtual
transverse and longitudinal plasmons with subsequent absorption of such
plasmons by the plasma. The emission of real plasmons dies out at $\omega \leq
\omega_p$, but the processes with virtual plasmons strongly enhance the
radiative decay. Applications of these results to radiative processes in white
dwarf cores and neutron star envelopes are discussed.
