Two classes of fast-declining type Ia supernovae

Fast-declining Type Ia supernovae (SN Ia) separate into two categories based on their bolometric and near-infrared (NIR) properties. The peak bolometric luminosity ($\mathrm{L_{max}}$), the phase of the first maximum relative to the optical, the NIR peak luminosity and the occurrence of a second maximum in the NIR distinguish a group of very faint SN Ia. Fast-declining supernovae show a large range of peak bolometric luminosities ($\mathrm{L_{max}}$ differing by up to a factor of $\sim$ 8). All fast-declining SN Ia with $\mathrm{L_{max}} < 0.3 \cdot$ 10$^{43}\mathrm{erg s}^{-1}$ are spectroscopically classified as 91bg-like and show only a single NIR peak. SNe with $\mathrm{L_{max}} > 0.5 \cdot$ 10$^{43}\mathrm{erg s}^{-1}$ appear to smoothly connect to normal SN Ia. The total ejecta mass (M$_{ej}$) values for SNe with enough late time data are $\lesssim$1 $M_{\odot}$, indicating a sub-Chandrasekhar mass progenitor for these SNe.