We study the innermost stable circular orbit ( ISCO ) of a test particle around rapidly rotating neutron stars .
Based on 12 different nuclear-matter equations of state ( EOS ) , we find numerically two approximately EOS-insensitive universal relations that connect the radius and orbital frequency of the ISCO to the spin frequency f and mass M of rotating neutron stars .
The relations are EOS-insensitive to about the 2 % level for a large range of Mf .
We also find that the universal relation for the ISCO radius agrees with the corresponding relation for the Kerr black hole to within 6 % up to Mf = 5000 M _ { \odot } { Hz } .
Our relations can be applied to accreting neutron stars in low-mass X-ray binaries .
Using the spin frequency f = 414 Hz and the highest kilohertz quasi-periodic oscillations ( kHz QPOs ) at 1220 Hz observed in the system 4U 0614+09 , we determine the mass of the neutron star to be 2.0 M _ { \odot } .
Our conclusion only makes a minimal assumption that the highest kHz QPO frequency is the ISCO frequency , bypassing the assumption of slow rotation and the uncertainty related to the dimensionless spin parameter , which are commonly required in the literature .