Quark Masses from Gaugino Condensation in String Theories

We present a mechanism able to generate the perturbatively absent up/down $<\mathbf{10}\cdot \mathbf{10}\cdot {\mathbf{5}}^H>$ quark Yukawa couplings of SU(5)/flipped SU(5) GUTS in Type II orientifold compactifications with D-branes. The mechanism works when there are Sp(N) gauge groups involved. The $\bar{\mathbf{5}}$'s get charged under the Sp(N) gauge groups and the generation of quark masses proceeds via the generation of the fermionic Sp(N) singlet condensate $<\bar{\mathbf{5}}\cdot \bar{\mathbf{5}}\cdot \bar{\mathbf{5}}\cdot \bar{\mathbf{5}}>$ in the term $(1/{M_s^5}) {\bf 10} \cdot {\bf 10} \cdot < {\bf {\bar 5} \cdot {\bar 5} \cdot {\bar 5} \cdot {\bar 5}}>$. Also non-chiral states charged under Sp gauge groups may become constrained by the requirement of Sp’s becoming strongly coupled.