Scattering Amplitudes and the CPT theorem in String Theory
  We discuss the role of the CPT transformation in first quantized string theories, both on the world-sheet and in the space-time. We explicitly show that the space-time CPT theorem holds for all first-quantized (perturbative) string theories in a Minkowski background of even dimension $D>2$. 
A. Pasquinucci and K. Roland