  The large collecting area of 4+ metre class telescopes allows atronomers to
observe bright objects on faster timescales than previously possible. In order
to exploit this capability, we designed a novel read-out mode for use on the
10-m Keck-II telescope on Mauna Kea, Hawaii. This took the form of a continuous
read-out mode, enabling us to obtain a target spectrum between 3500 and 8500
\AA, with a dispersion of ~2.5 \AA/pix, every *70 milliseconds*. I present
results from 5 nights of observations of Interacting Binaries, using this novel
data acquisition system in July 1998. During the run we obtained more than 1.2
million optical spectra, many of which were simultaneous with pointed RXTE
observations. This fast spectroscopic capability has allowed us to explore
regimes previously only accessible with broadband instruments. I will
concentrate on results from the coordinated Keck/RXTE observations, which have
revealed interesting correlated variability on timescales from seconds to days.
I will also show the results of a spectroscopic-timing analysis of QPOs in
Hercules X-1, which shows evidence for the decay of individual blobs of
material orbiting the neutron star.
