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We present calculations of the final phase of inspiral of irrotationalstrange star binaries. Two types of equation of state at zero temperature areused - the MIT bag model and the Dey et al. 1998 model of strange quark matter.We study the precoalescence stage within the Isenberg-Wilson-Mathewsapproximation of General Relativity using a multidomain spectral method. Thegravitational-radiation driven evolution of the binary system is approximatedby a sequence of quasi-equilibrium configurations at a fixed baryon number andwith decreasing separation. We find that the innermost stable circular orbit(ISCO) is determined always by an orbital instability for binaries consistingof two stars built predominantly of strange quark matter independently on thetotal mass of a binary system and compactness parameter of each star. Incontrast, for neutron stars described by baryonic equation of state withoutexotic phases the ISCO is given by the mass-shedding limit. The gravitationalwave frequency at the ISCO, which marks the end of the inspiral phase, isalways higher than 1.1kHz for equal masses irrotational strange quark starswith the total mass-energy of a binary system greater than $2 M_\odot$. We findthat the dependence of the frequency of gravitational waves at the ISCO on thecompactness parameter for the equal mass binaries can be described by the samesimple analytical formulae for broad ranges of masses independently on astrange star model. Detailed comparisons with binary neutrons star models, aswell as with the third order Post-Newtonian point-mass binaries are given. Thedifference in the phase, for two $1.35 M_\odot$ strange stars, between ournumerical results and 3PN is $\sim 40 %$ for the last two orbits of inspiral.

The final phase of inspiral of strange quark star binaries