Two classes of fast-declining Type Ia supernovae

We aim to characterise a sample of fast-declining Type Ia supernovae (SN Ia) using their bolometric and near-infrared (NIR) properties. Based on these properties, we find that fast-declining SN Ia separate into two categories based on their bolometric and NIR properties. The peak bolometric luminosity (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 (L-max differing by up to a factor of similar to 8). All fast-declining SN Ia with L-max < 0 : 3 x 1043 erg s 1 are spectroscopically classified as 91bg-like and show only a single NIR peak. SNe with L-max > 0 : 5 x 10(43) erg s(-1) appear to smoothly connect to normal SN Ia. The total ejecta mass (Mej) values for SNe with enough late time data are less than or similar to 1 M-circle dot, indicating a sub-Chandrasekhar mass progenitor for these SNe.