  We report on Rossi X-ray Timing Explorer observations of four type I X-ray
bursters; namely 1E1724-3045, GS1826-238, SLX1735-269 and KS1731-260. The first
three were in a low state (LS) whereas KS1731-260 was in a high state. The LS
sources have very similar power spectra, displaying high frequency noise up to
\~200 Hz. For KS1731-260, its power spectrum is dominated by noise at
frequencies below 20 Hz. In addition a quasi-periodic oscillation at 1200 Hz is
detected. The 1-200 keV LS spectra are all consistent with resulting from
thermal Comptonization with an electron temperature (kTe) around 25-30 keV. For
KS1731-260, the spectrum is also dominated by Comptonization, but with kTe
around 3 keV and no significant hard X-ray emission. With the exception of
GS1826-238, there is an underlying soft component. For all sources, we have
detected an iron line at 6.4 keV. A reflection component is present in the
spectra of GS1826-238 and SLX1735-269. We suggest a model in which the region
of main energy release, where hard X-rays are produced would be an optically
thin boundary layer merged with an Advection Dominated Accretion Flow (ADAF).
The soft component observed probably represents the unscattered emission from
the accretion disk of variable inner radius. When the accretion rate increases,
the inner disk radius shrinks, the strength of the reflected component and
associated iron line increase, and the Comptonization region cools off in
response to an increased cooling flux from the accretion disk and from the
reprocessed/reflected component. Finally, in the light of these observations,
we discuss extensively the various criteria recently proposed to distinguish
between non-quiescent accreting black holes and neutron stars.
