I think it would still be considered OT. Of course one could keep crossing back to T until you might as well call it a trumpet but as long as there is O there I believe the hybridizers keep calling it OT.
I did make a list of crosses that I have made that produced seeds with embryos. Since I can't possibly start all of my own seeds I depend on others to report on success (or lack thereof).
I have Flugelhorn strain (white, 4n) x Rococo in the NALS sale now. It is inaccurately listed in the Division 6 Trumpet group. That may be my fault. It is the only T x OT cross I did last year and it made a fair amount of seeds with embryos.
