Borisov, Galin [P:(DE-Juel1)161221] ; Avramov, S. [P:(DE-HGF)0] ; Petkucheva, E. [P:(DE-HGF)0] ; Lefterova, E. [P:(DE-HGF)0] ; Slavcheva, E. [P:(DE-HGF)0] ; Lehnert, Werner [P:(DE-Juel1)129883]