Columbia Univ, Columbia Astrophys Lab, New York, NY 10027 USA
; Inst High Energy Phys, LCRHEA, Beijing 100039, Peoples R China
; Univ Space Res Assoc, Huntsville, AL 35806 USA
; NASA, George C Marshall Space Flight Ctr, Huntsville, AL 35812 USA
We present correlations between the frequency of the fast quasi-periodic oscillations (QPOs) and the shape of the X-ray spectrum in the atoll sources 4U 1608-52 and 4U 0614+091. We find that the photon index of a power-law component of the energy spectrum is well correlated with the QPO frequency in both sources. The correlation between the photon index and the QPO frequency most likely represents a common dependence on some physical parameter of the system.