Quark masses and mixing angles in heterotic orbifold models
  We study systematically the possibility for realizing realistic values of quark mass ratios $m_c/m_t$ and $m_s/m_b$ and the mixing angle $V_{cb}$ by using only renormalizable Yukawa couplings derived from heterotic orbifold models. We assume one pair of up and down sector Higgs fields. We show realistic examples including hierarchical and democratic forms of Yukawa matrices. 
Pyungwon Ko, Tatsuo Kobayashi and Jae-hyeon Park