Correlations discovered between millisecond timing properties and spectral
properties in neutron star x-ray binaries are described and then interpreted in
relation to accretion flows in the systems. Use of joint timing and spectral
observations to test for the existence of the marginally stable orbit, a key
prediction of strong field general relativity, is described and observations of
the neutron star x-ray binary 4U1820-303 which suggest that the signature of
the marginally stable orbit has been detected are presented.Comment: 10 pages, Invited talk to appear in the Proceedings of the Conference
X-ray Astronomy '999: Stellar Endpoints, AGNs and the Diffuse X-ray
Backgroun
