We would like to thank Peter Eggleton, Douglas Heggie and Koji Takahashi for many stimulating discussions. This work was supported in part by the Netherlands Organization for Scientific Research (NWO) under grant PGS 78-277 and SIR 13-4109 and by the Leids Kerkhoven Boscha Fonds. SPZ thanks the Institute for Advanced Study and the University of Tokyo for their hospitality. Edward P.J. van den Heuvel of the Astronomical Institute "Anton Pannekoek" is acknowledged for financial support and for inviting our group for an extended work visit. This investigation is supported by Spinoza grant 08-0 to E. P. J. van den Heuvel.
© European Southern Observatory (ESO) 1998
Online publication: August 17, 1998