The Ramurans were a warp-capable species from Ramura in the Delta Quadrant. Physically, they were humanoids who released a unique pheromone. This affected other species, preventing them from remembering encounters with the Ramurans for more than a few hours. Due to technological enhancements, they were also immune to most forms of scanning technology (including tricorders). All record of her was wiped by the tracers, but Chakotay used a pen and paper to record his experiences before the memories faded. (VOY: "Unforgettable")