  We present a first systematic comparison of superluminous Type Ia supernovae
(SNe Ia) at late epochs, including previously unpublished photometric and
spectroscopic observations of SN 2007if, SN 2009dc and SNF20080723-012.
Photometrically, the objects of our sample show a diverse late-time behaviour,
some of them fading quite rapidly after a light-curve break at ~150-200d. The
latter is likely the result of flux redistribution into the infrared, possibly
caused by dust formation, rather than a true bolometric effect. Nebular spectra
of superluminous SNe Ia are characterised by weak or absent [Fe III] emission,
pointing at a low ejecta ionisation state as a result of high densities. To
constrain the ejecta and 56Ni masses of superluminous SNe Ia, we compare the
observed bolometric light curve of SN 2009dc with synthetic model light curves,
focusing on the radioactive tail after ~60d. Models with enough 56Ni to explain
the light-curve peak by radioactive decay, and at the same time sufficient mass
to keep the ejecta velocities low, fail to reproduce the observed light-curve
tail of SN 2009dc because of too much gamma-ray trapping. We instead propose a
model with ~1 solar mass of 56Ni and ~2 solar masses of ejecta, which may be
interpreted as the explosion of a Chandrasekhar-mass white dwarf (WD)
enshrouded by 0.6-0.7 solar masses of C/O-rich material, as it could result
from a merger of two massive C/O WDs. This model reproduces the late light
curve of SN 2009dc well. A flux deficit at peak may be compensated by light
from the interaction of the ejecta with the surrounding material.
