  Near-infrared spectra of the subluminous Type Ia SN1999by are presented which
cover the time evolution from about 4 days before to 2 weeks after maximum.
Analysis of these data was accomplished through the construction of an extended
set of delayed detonation (DD) models. The explosion, light curves and spectra
including their evolution with time are calculated consistently leaving the
initial WD and the description of the nuclear burning front the free
parameters. We cover the entire range of normal to subluminous SNeIa. From this
model set, one was selected for SN99by by matching properties of the synthetic
& observed optical light curves. We find DD models require a certain amount of
burning during the deflagration phase setting a lower limit for the absolute
brightness. For SN1999by, a model close to the minimum 56Ni production is
required. Without tuning,good agreement has been found between synthetic and
observed IR spectra. In contrast to 'normal' SNeIa and prior to maximum, the
NIR spectra of SN1999by are dominated by products of explosive carbon burning.
Spectra taken after maximum are dominated by products of incomplete Si burning.
Pure deflagration scenarios or mergers are unlikely. However,problems for DD
models still remain, as the data seem to be at odds with recent predictions
from 3-D models which find significant mixing of the inner layers. Possible
solutions include the effects of rapid rotation on the propagation of nuclear
flames, or extensive burning of carbon just prior to the runaway.
