In 1998, Wijnands and his PhD advisor Michiel van der Klis discovered coherent millisecond X-ray pulsations in the persistent flux of the X-ray binary SAX J1808.4-3658. This was strong evidence for the model in which radio millisecond pulsars are spun up to periods of a few milliseconds by the accretion of matter from a companion star in an X-ray binary.
