Thank you Ben.
I tried, but unfortunately a "small" problem came up. I managed to take new blue subs but after integration the stars were just a little bit smaller. Actually this was the first time I took RGB subs with my little scope and the reducer: this was mainly my "large emmision nebula setup". Looks to me that I ran into the classic blue bloat problem: I use Baader filters and the blue cuts off somwhere at ~380nm. That's probably to much (or less...) for my scope.
I'm not sure if I end this litte RGB experiment or try an other blue fliter. The Astronomik deep sky blue filter seems to fit better (cut of at ~420nm), but the filter is 1mm thick. This would change my backfocus because the Baaders are 2mm thick. Hmmm.