The Zero Tension Limit of The Virasoro Algebra and the Central Extension
  We argue that the Virasoro algebra for the closed bosonic string can be cast in a form which is suitable for the limit of vanishing string tension. In this form the limit of the Virasoro algebra gives the null string algebra. The anomalous central extension is seen to vanish as well when $T\to 0$. 
F. Lizzi (Napoli)