N=2 Strings on Orbifolds
  We study closed N=2 strings on orbifolds of the form T^4/Z_2 and C^2/Z_2. We compute the torus partition function and prove its modular invariance. We analyse the BRST cohomology of the theory, construct the vertex operators, and compute three and four point amplitudes of twisted and untwisted states. We introduce a background of D-branes, and compute twist states correlators. 
Dan Gluck (1), Yaron Oz (1) and Tadakatsu Sakai (2) ((1) Tel-Aviv   University, Israel, (2) Ibaraki University, Japan)