1999A&A...344L..71B

It has been suggested that low mass X-ray binaries (LMXBs) contain strange (quark) stars. We show that the properties of bulk strange matter are severely constrained if the recently discovered kilohertz quasi periodic oscillations (QPOs) of the X-ray flux from LMXBs occur at orbital frequencies around strange stars. In particular, the simplest equation of state of strange matter is ruled out if the observed saturation (at 1.07kHz) of the QPO frequency in 4U 1820-30 occurs at the orbital frequency either just outside the surface of the strange star or in the innermost (marginally) stable orbit allowed by general relativity.