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\bibitem{r}P. Roberts, \PL{B244} (1990) 429.
\bibitem{ciz1}    A. Cappelli, C.Itzykson and J.-B. Zuber,
	\NP{B280} (1987) 445. 
\bibitem{gq}D. Gepner and Z. Qiu,
 \NP{B285} (1987) 423.
\bibitem{ciz2}    A. Cappelli, C.Itzykson and J.-B. Zuber,
	\CM{113} (1987) 1. 
\bibitem{k}	A. Kato,
	\MPL{A2} (1987) 585.
\bibitem{b1}D. Bernard,
 \NP{B288} (1987) 628.
\bibitem{alz}D. Altsch\"{u}ler, J. Lacki and Ph. Zaugg,
 \PL{B205} (1988) 281.
\bibitem{b2}P. Bouwknegt,
 \NP{B290} (1988) 507.
\bibitem{cr}	P. Christe and F. Ravanini,
	\JMP{A4} (1989) 897.
\bibitem{sy}	A. N. Schellekens and S. Yankielowics,
	{\em Simple Currents, Modular Invariants and Fixed Points}, CERN Preprint,
	CERN-TH.5622/90 (1989); \NP{B327} (1989) 673.
\bibitem{ms}G. Moore and N. Seiberg,
 \NP{B313} (1989) 41.
\bibitem{w}N. P. Warner,
	\CM{130} (1990) 205.
\bibitem{bi}	M. Bauer and C. Itzykson,
	\CM{127} (1990) 617.
\bibitem{gw}	D. Gepner and E. Witten
	\NP{B278} (1986) 493.
\bibitem{gko}	P. Goddard, A. Kent and D. Olive,
	\CM{103} (1986) 105.
\bibitem{lsw}	W. Lerche, A. N. Schellekens and N. Warner,
	\PR{177} (1989) 1. 
\bibitem{go}P. Goddard and D. Olive, in {\em Vertex
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\end{thebibliography}
