arXiv:hep-th/0305098v3 16 Jan 2004 UUITP-08/03 hep-th/0305098 Tensionless Strings, WZW Models at Critical Level and Massless Higher Spin Fields UlfLindstr¨om a 1 and Maxim Zabzine b 2 a Department of Theoretical Physics, Uppsala University, Box 803, SE-751 08 Uppsala, Sweden b INFN Sezione di Firenze, Dipartimento di Fisica Via Sansone 1, I-50019 Sesto F.no (FI), Italy ABSTRACT We discuss the notion of tensionless limit in quantum bosonic string theory, especially in flat Minkowski space, noncompact group manifolds (e.g., SL(2,R)) and coset manifolds (e.g., AdS d ). We show that in curved space typically there exists a critical value of the tension which is related to the critical value of the level of the corresponding affine algebra. We argue that at the critical level the string theory becomes tensionless and that there exists a huge new symmetry of the theory. We discuss the appearance of the higher spin massless states at the critical level. 1 e-mail address: ulf.lindstrom@teorfys.uu.se 2 e-mail address: zabzine@fi.infn.it; Address after September 1, 2003: LPTHE, Universit´ es Paris VI et VII, 4pl Jussieu, 75252 Paris, Cedex 05 France 1