  Theoretically, the phase transition between the confined and deconfined
phases of quarks can have a remarkable effect on the spin properties of
millisecond pulsars and on the spin distribution of the population of x-ray
neutron stars in low-mass binaries. In the latter class of stars, the effect
has already been observed---a strong clustering in the population in a narrow
band of spins. The observed clustering cannot presently be uniquely assigned to
the phase transition as cause. However, there is another possible signal---not
so far observed---in millisecond pulsars that we also discuss. It would have
the same origin, and its discovery would tend to confirm the interpretation in
terms of a phase transition in the stellar core and that the high density phase
exists in the higer mass canonical neutron stars.
