  It has been suggested that the frequency in the co-rotating innermost stable
circular orbit (ISCO) about a compact stellar remnant can be determined through
X-ray observations of low-mass X-ray binaries, and that its value can be used
to constrain the equation of state of ultradense matter.
  Upon constructing numerical models of rapidly rotating strange (quark) stars
in general relativity, we find that for stars rotating at the equatorial
mass-shedding limit, the ISCO is indeed above the stellar surface, for a wide
range of central energy densities at a height equal to 11% of the
circumferential stellar radius, which scales inversely with the square root of
the energy density, of self-bound quark matter at zero presure. In contrast to
static stars, the ISCO frequencies of rapidly rotating strange stars can be as
low as 0.9 kHz for a 1.3 solar mass strange star. Hence, the presence of
strange stars in low-mass X-ray binaries cannot be excluded on the basis of the
currently observed frequencies of kHz QPOs, such as the cut-off frequency of
1066 Hz in 4U 1820-30.
