Abstract. Strong constraints on the coupling of new light particles to the Standard Model (SM) arise from their production in the hot cores of stars, and the effects of this on stellar cooling. The large electron density in stellar cores significantly modifies the in-medium propagation of SM states. I will show that for new light particles which have an effective in-medium mixing with the photon, such plasma effects can result in parametrically different production rates to those obtained from a naive calculation. Taking these previously-neglected contributions into account, I will give updated estimates for the stellar cooling bounds on a number of light new particle candidates. In particular, the bounds on light (m < keV) scalars coupling to electrons or nucleons are strengthened by 3 orders of magnitude, supernova cooling bounds on dark photon couplings are significantly revised, and the mass dependence of stellar bounds on new vectors are qualitatively changed.