  We study the spin evolution of X-ray neutron stars in binary systems, which
are being spun up by mass transfer from accretion disks. Our investigation
reveals that a quark phase transition resulting from the changing central
density induced by the changing spin, can lead to a pronounced peak in the
frequency distribution of X-ray neutron stars. This finding provides one of
several possible explanations available in the literature, or at least a
contributor to part of the observed anomalous frequency distribution of neutron
stars in low-mass X-ray binaries (LMXBs), which lie in a narrow band centered
at about 300 Hz, as found by the Rossi Explorer (RXTE).
