  We use archival data from the Rossi X-Ray Timing Explorer to examine 125 type
I X-ray bursts from the 9 weakly magnetic accreting neutron stars where
millisecond oscillations have been detected during some bursts. We find that
oscillations from the 6 "fast" (approximately 600 Hz) sources are almost always
observed during radius expansion bursts, whereas oscillations from the 3 "slow"
(about 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.
