1. H. Faissner et al., Phys. Lett. B 125, 230 (1983)
2. E. Isiksal, D. Rein, J.G. Morfin, Phys. Rev. Lett. 52, 1096 (1984)
3. H.J. Grabosch et al. (SKAT Collaboration), Zeit. Phys. C 31, 203 (1986)
4. F. Bergsma et al. (CHARM Collaboration), Phys. Lett. B 157, 469 (1985)
5. P. Vilain et al. (CHARM-II Collaboration), Phys. Lett. B 313, 267 (1993)