  The recently discovered FeAs-based materials exhibit a $(\pi,0)$ Spin Density
Wave (SDW) in the undoped state, which gives way to superconductivity upon
doping. Here we show that due to an interesting topological feature of the band
structure, the SDW state cannot acquire a full gap. This is demonstrated within
the SDW mean-field theory of both a simplified two band model and a more
realistic 5-band model. The positions of the nodes are different in the two
models and can be used to detected the validity of each model.
