  We present an analysis of ultraviolet (UV) to near-infrared observations of
the fast-declining Type Ia supernovae (SNe Ia) 2007on and 2011iv, hosted by the
Fornax cluster member NGC 1404. The B-band light curves of SN 2007on and SN
2011iv are characterised by dm_15(B) decline-rate values of 1.96 mag and 1.77
mag, respectively. Although they have similar decline rates, their peak B- and
H-band magnitudes differ by ~0.60 mag and ~0.35 mag, respectively. After
correcting for the luminosity vs. decline rate and the luminosity vs. colour
relations, the peak B-band and H-band light curves provide distances that
differ by ~14% and ~9%, respectively. These findings serve as a cautionary tale
for the use of transitional SNe Ia located in early-type hosts in the quest to
measure cosmological parameters. Interestingly, even though SN 2011iv is
brighter and bluer at early times, by three weeks past maximum and extending
over several months, its B-V colour is 0.12 mag redder than that of SN 2007on.
To reconcile this unusual behaviour, we turn to guidance from a suite of
spherical one-dimensional Chandrasekhar-mass delayed-detonation explosion
models. In this context, 56Ni production depends on both the so-called
transition density and the central density of the progenitor white dwarf. To
first order, the transition density drives the luminosity-width relation, while
the central density is an important second-order parameter. Within this
context, the differences in the B-V color evolution along the Lira regime
suggests the progenitor of SN~2011iv had a higher central density than
SN~2007on.
