On the instability of 3d null singularities

String propagation on a three-dimensional lorentzian string orbifold with a null singularity has been studied by Horowitz and Steif, and more recently by Liu, Moore and Seiberg. We analyze the target space as a classical gravitational background. The singularity becomes spacelike when an arbitrarily small amount of matter is thrown at the singularity. This can be seen directly by constructing a Vaidya-type solution, or by studying the null singularity as a limit of the M=0, J=0 BTZ black hole metric. (author)