S duality at the black hole threshold in gravitational collapse.

We study gravitational collapse of the axion/dilaton eld in classical low energy string theory, at the threshold for black hole formation. A new critical solution is derived that is spherically symmetric and continuously self-similar. The universal scaling and echoing behavior discovered by Choptuik in gravitational collapse appear in a somewhat di erent form. In particular, echoing takes the form of SL(2;R) rotations (cf. S-duality). The collapse leaves behind an outgoing pulse of axion/dilaton radiation, with nearly but not exactly at spacetime within it. Typeset using REVTEX Electronic address: [email protected] Electronic address: [email protected] Electronic address: [email protected]