We analyzed the recently published kHz QPO data in the neutron star low-mass X-ray binaries ( LMXBs ) , in order to investigate the different correlations of the twin peak kilohertz quasi-periodic oscillations ( kHz QPOs ) in bright Z sources and in the less luminous Atoll sources .
We find that a power-law relation \nu _ { 1 } \sim \nu _ { 2 } ^ { b } between the upper and the lower kHz QPOs with different indices : b \simeq 1.5 for the Atoll source 4U 1728-34 and b \simeq 1.9 for the Z source Sco X-1 .
The implications of our results for the theoretical models for kHz QPOs are discussed .