The peculiar QSO J0927+2943 shows multiple sets of emission lines in its optical spectrum. This signature has been interpreted as the relative motion between a black hole, either recoiling or bound in a binary system, and its host galaxy, or as a superposition of two galaxies along the line of sight. In order to test these scenarios, we have collected 2mm CO(2-1) observations using the IRAM Plateau de Bure Interferometer, and optical images and spectroscopy at the Calar Alto observatory. Together with archival HST images, these data provide unique insights on the nature of this system. The recoiling/binary black hole scenarios are ruled out by the clear detection of a galactic-scale molecular gas reservoir at the same redshift of the QSO broad lines. The observations presented here also disfavour the superposition model, although with less constraints. Thus, the origin of the second, bright set of narrow emission lines in J0927+2943 is still unknown.
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