Matrix Orientifolding and Models with Four or Eight Supercharges
  The conditions under which matrix orientifolding and supersymmetry transformations commute are known to be stringent. Here we present the cases possessing four or eight supercharges upon ${\bf Z}_3$ orbifolding followed by matrix orientifolding. These cases descend from the matrix models with eight plus eight supercharges. There are fifty in total, which we enumerate. 
Hiroshi Itoyama, Reiji Yoshioka