Results from the paper

  Yuichiro Kiyo, Dirk Seidel, Matthias Steinhauser:
  ${\cal O}(\alpha\alpha_s)$ corrections to the $\gamma t\bar{t}$ vertex at the top quark threshold
  TTP08-43 

(see also: 
http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/webmaster/abs2html?ttp08-43)