The top Yukawa coupling from 10D supergravity with E_8 x E_8 matter
  We consider the compactification of N=1, D=10 supergravity with E_8 x E_8 Yang-Mills matter to N=1, D=4 model with 3 generations. With help of embedding SU(5)->SO(10)->E_6->E_8 we find the value of the top Yukawa coupling $\lambda_t=\sqrt{16\pi\alpha_{GUT}/3}$ at the GUT scale. 
Konstantin Zyablyuk