An investigation has been undertaken of two optical systems encountered in electron irradiated n- and p-type 4H SiC when studied by photoluminescence microscopy. These two systems anneal out together in the temperature interval 300-400degreesC. The vibronic structures of the optical systems have similar gap modes and local vibrational modes but different polarization behaviour. It is argued that they are related to carbon interstitials which become mobile at relatively low temperatures.