If I understood this correctly, the white dwarf creating this supernova was 0.27 solar mass, but collided with something else in order to explode.

https://arxiv.org/abs/1903.08665

ASASSN-18tb: A Most Unusual Type Ia Supernova Observed by TESS and SALT

P. J. Vallely, et al. (Submitted on 20 Mar 2019)

We present photometric and spectroscopic observations of the unusual Type Ia supernova ASASSN-18tb, including a series of SALT spectra obtained over the course of nearly six months and the first observations of a supernova by the Transiting Exoplanet Survey Satellite (TESS). We confirm a previous observation by Kollmeier et al. (2019) showing that ASASSN-18tb is the first relatively normal Type Ia supernova to exhibit clear broad (∼1000 km s −1) Hα emission in its nebular phase spectra. We find that this event is best explained as a sub-Chandrasekhar mass explosion with M Ni ≈ 0.27 M⊙. Despite the strong Hα signature at late times, we find that the early rise of the supernova shows no evidence for deviations from a single-component power-law and is best fit with a moderately shallow power-law of index 1.400.03 . We find that the Hα luminosity remains approximately constant after its initial detection at phase +37 d, and that the Hα velocity evolution does not trace that of the Fe~III λ4660 emission. These suggest that the Hα emission arises from circumstellar medium (CSM) rather than swept up material from a non-degenerate companion. However, ASASSN-18tb is strikingly different from other known CSM-interacting Type Ia supernovae in a number of significant ways. Those objects typically show an Hα luminosity two orders of magnitude higher than what is seen in ASASSN-18tb, pushing them away from the empirical light-curve relations that define "normal" Type Ia supernovae. Conversely, ASASSN-18tb exhibits a fairly typical light curve and luminosity for an underluminous or transitional SN Ia, with M R ≈−18.1 mag. Moreover, ASASSN-18tb is the only SN Ia showing Hα from CSM interaction to be discovered in an early-type galaxy.