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While Chandrasekhar-mass ($M_{\rm Ch}$) models with a low $^{56}\rm{Ni}$yield can match the peak luminosities of fast-declining, 91bg-like Type Iasupernovae (SNe Ia), they systematically fail to reproduce their fasterlight-curve evolution. Here we illustrate the impact of a low ejecta mass onthe radiative display of low-luminosity SNe Ia, by comparing a sub-$M_{\rm Ch}$model resulting from the pure central detonation of a C-O White Dwarf (WD) to a$M_{\rm Ch}$ delayed-detonation model with the same $^{56}\rm{Ni}$ yield of0.12 M$_{\odot}$. Our sub-$M_{\rm Ch}$ model from a 0.90 M$_{\odot}$ WDprogenitor has a $\sim$5 day shorter rise time in the integrated UV-optical-IR(uvoir) luminosity, as well as in the $B$-band, and a $\sim$20 per cent higherpeak uvoir luminosity ($\sim$1 mag brighter peak $M_B$). This sub-$M_{\rm Ch}$model also displays bluer maximum-light colours due to the larger specificheating rate, and larger post-maximum uvoir and $B$-band decline rates. Theluminosity decline at nebular times is also more pronounced, reflecting theenhanced escape of gamma rays resulting from the lower density of theprogenitor WD. The deficit of stable nickel in the innermost ejecta leads to anotable absence of forbidden lines of [Ni II] in the nebular spectra. Incontrast, the $M_{\rm Ch}$ model displays a strong line due to [Ni II] 1.939$\mu\rm{m}$, which could in principle serve to distinguish between differentprogenitor scenarios. Our sub-$M_{\rm Ch}$ model offers an unprecedentedagreement with optical and near-infrared observations of the 91bg-like SN1999by, making a strong case for a WD progenitor significantly below theChandrasekhar-mass limit for this event and other low-luminosity SNe Ia.

The detonation of a sub-Chandrasekhar-mass white dwarf at the origin of the low-luminosity Type Ia supernova 1999by