  We study electromagnetically induced transparency (EIT) of a weakly
interacting cold Rydberg gas. We show that the onset of interactions is
manifest as a depopulation of the Rydberg state and numerically model this
effect by adding a density-dependent non-linear term to the optical Bloch
equations. In the limit of a weak probe where the depopulation effect is
negligible, we observe no evidence of interaction induced decoherence and
obtain a narrow Rydberg dark resonance with a linewidth of <600 kHz, limited by
the Rabi frequency of the coupling beam
