OK i see. Just to make sure i understood this correctly. I rename the file as above. I load both in the library. I put the fill file only in the group, but not the _a file, that just stays in the library.
Than Caspar will automatically see that alpha file when i play the fill file on the key+fill channel. Is that correct?
Yes, that is correct.
This did the trick all works perfect. Thank you very much for the persistence and the help!