I have in the past tried to find when SEPP stopped its activities, and when IMPS started, to be better able to date Smurfy items with those imprints. The conclusion then was that IMPS was active as early as 1988, and SEPP was definitely still active in 1986, and that some other clues gave the impression that they had existed simultaneously for a while.
I can now say with near certainty that IMPS was founded in 1984 by Veronique Culliford, the daughter of Peyo, and that thus IMPS and SEPP have definitely existed together for a few years at least. I still don't know when SEPP stopped existing (or stopped working for Peyo, as they also produced items for other artists). This may be 1989, when Peyo quit his editor Dupuis, which seemed to be affiliated with SEPP.