1. A. Schild, Classical null strings, Phys. Rev.D 16 (1977) 1722 [INSPIRE].
2. J. Isberg, U. Lindström, B. Sundborg and G. Theodoridis, Classical and quantized tensionless strings, Nucl. Phys.B 411 (1994) 122 [hep-th/9307108] [INSPIRE].
3. A. Bagchi, Tensionless strings and Galilean conformal algebra, JHEP05 (2013) 141 [arXiv:1303.0291] [INSPIRE].
4. A. Bagchi, S. Chakrabortty and P. Parekh, Tensionless strings from worldsheet symmetries, JHEP01 (2016) 158 [arXiv:1507.04361] [INSPIRE].
5. H. Bondi, M.G.J. van der Burg and A.W.K. Metzner, Gravitational waves in general relativity. 7. Waves from axisymmetric isolated systems, Proc. Roy. Soc. Lond.A 269 (1962) 21 [INSPIRE].