Nonthermal nature of extremal Kerr black holes

Liberati, Rothman and Sonego have recently showed that objects collapsing into extremal Reissner-Nordstrom black holes do not behave as thermal objects at any time in their history. In particular, a temperature, and hence thermodynamic entropy, are undefined for them. I demonstrate that the analysis goes through essentially unchanged for bodies collapsing into extremal Kerr black holes. (C) 2000 Published by Elsevier Science B.V.