An extended N=1 fermionic supercurrent and its associated N=2 superconformal algebra

An extended free fermionic construction of the internal N = 1 worldsheet supercurrent for four-dimensional superstring theory is given. We show how it can describe theories with massless fermions, and we discuss the corresponding N = 2 superconformal algebra. As an intermediate step, we show that an internal N = 2 global superconformal invariance occurs in any superstring theory with massless fermions at tree level. To demonstrate this fact, we give the N = 2 supercurrents for a model with N = 1 spacetime supersymmetry and a model without spacetime supersymmetry.