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As a candidate 'super-Chandrasekhar' or 09dc-like Type Ia supernova (SN Ia),SN 2012dn shares many characteristics with other members of this remarkableclass of objects but lacks their extraordinary luminosity. Here, we present anddiscuss the most comprehensive optical data set of this SN to date, comprisedof a densely sampled series of early-time spectra obtained within the NearbySupernova Factory project, plus photometry and spectroscopy obtained at the VLTabout 1 yr after the explosion. The light curves, colour curves, spectral timeseries and ejecta velocities of SN 2012dn are compared with those of other09dc-like and normal SNe Ia, the overall variety within the class of 09dc-likeSNe Ia is discussed, and new criteria for 09dc-likeness are proposed.Particular attention is directed to additional insight that the late-phase dataprovide. The nebular spectra show forbidden lines of oxygen and calcium,elements that are usually not seen in late-time spectra of SNe Ia, while theionisation state of the emitting iron plasma is low, pointing to low ejectatemperatures and high densities. The optical light curves are characterised byan enhanced fading starting ~60 d after maximum and very low luminosities inthe nebular phase, which is most readily explained by unusually early formationof clumpy dust in the ejecta. Taken together, these effects suggest a stronglyperturbed ejecta density profile, which might lend support to the idea that09dc-like characteristics arise from a brief episode of interaction with ahydrogen-deficient envelope during the first hours or days after the explosion.

SN 2012dn from early to late times: 09dc-like supernovae reassessed