Gauge Coupling Unification and the Top Mass in String Models with $SU(4)\times O(4)$ Symmetry

We discuss the low energy implications of gauge coupling unification at the string scale, taking into account string threshold corrections in the $SU(4) {\times} O(4)$ model. We express $sin^2\theta_W$ and $a_3$ as functions of the calculable string threshold differences and discuss simple examples of spectra which retain the successful predictions of the supersymmetric unification. Using further the low energy data and reasonable values of the common gauge coupling at the string scale, we obtain the range of the threshold corrections. Finally, we study the top Yukawa coupling ($h_t$) evolution whose initial value is determined in terms of the common gauge coupling at the string scale. We find that $h_t$ reaches its (quasi) infra-red fixed point at the weak scale and discuss the implications on the top mass.