Doubled strings, negative strings and null waves

Understand

We revisit the fundamental string (F1) solution in the doubled formalism.

• We show that the wave-like solution of double field theory (DFT) corresponding to the F1/pp-wave duality pair is more properly a solution of the DFT action coupled to a doubled sigma model action.
• The doubled string configuration which sources the pp-wave can be thought of as static gauge with the string oriented in a dual direction.
• We also discuss the DFT solution corresponding to a vibrating string, carrying both winding and momentum.