We thank Jan van Paradijs for reading the manuscript. This work was partially supported through NWO Spinoza grant 08-0 to E.P.J. van den Heuvel and RFBR Grant 96-02-16351. L.R.Y acknowledges hospitality of the Astronomical Institute "Anton Pannekoek" and Meudon Observatory.
© European Southern Observatory (ESO) 1998
Online publication: March 30, 1998