Millisecond Oscillations and Photospheric Radius Expansion in Thermonuclear X-Ray Bursts

We use archival data from the Rossi X-Ray Timing Explorer to examine 125 type I X-ray bursts from the nine weakly magnetic accreting neutron stars where millisecond oscillations have been detected during some bursts. We find that oscillations from the six ``fast'' (~=600 Hz) sources are almost always observed during radius expansion bursts whereas oscillations from the three ``slow'' (~=300 Hz) sources are about equally likely to be found in bursts both with and without photospheric radius expansion. This strongly suggests that the distinction between these two source groups cannot be an observational selection effect but must instead arise from some physical mechanism.