Zero-phonon lines are found at 732.7 nm in CaF₂ and 835.0 nm in SrF₂ when additively coloured crystals containing F₃ centres are exposed to ionizing radiations at low temperatures. The centres giving rise to these lines are not stable at room temperature. The maximum obtainable concentration of the centres is proportional to the number of F₃ centres present in the crystals. Measured uniaxial stress and electric field effects are similar to those found for F₃ centres. It is suggested that the new centres are negatively charged F₃ centres (i.e. F₃^-).