The BRST Operator for the Large $N=4$ Superconformal Algebra
  We review the detailed structure of the large $N=4$ superconformal algebra, and construct its BRST operator which constitutes the main object for analyzing $N=4$ strings. We then derive the general condition for the nilpotency of the BRST operator and show that there exists a line of critical $N=4$ string theories. 
Fiorenzo Bastianelli and Nobuyoshi Ohta