  We analysed RXTE data of Sco X-1 which show kHz quasi-periodic oscillations
(QPOs) and the $\sim$6--8 Hz normal-branch oscillation (NBO) simultaneously.
Using power spectra of 0.03--0.5 s data segments, we find that both the upper
kHz QPO frequency $\nu_2$ and the ratio of lower to upper kHz QPO amplitude are
anticorrelated to variations in the X-ray count-rate taking place on the NBO
time scale. The frequency dependence is similar to (but probably weaker than)
that found on longer time scales, but the power ratio dependence is opposite to
it. A model where radiative stresses on the disk material, modulated at the NBO
frequency, lead to changes in $\nu_2$ can explain the data; this implies some
of the NBO flux changes originate from inside the inner disk radius. We discuss
how these findings affect our understanding of kHz QPOs and of the
low-frequency variability of low-mass X-ray binaries.
