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As part of the 100IAS survey, a program aimed to obtain nebular-phase spectrafor a volume-limited and homogeneous sample of Type Ia supernovae (SNe Ia), weobserved ASASSN-18tb (SN 2018fhw) at 139 days past maximum light. ASASSN-18tbwas a fast-declining, sub-luminous event that fits well within the observedphotometric and spectroscopic distributions of the SN Ia population. We detecta prominent H$\alpha$ emission line of $L_{{\rmH}\alpha}=2.2\pm0.2\times10^{38}$ ergs s$^{-1}$ with FWHM $\approx1100$ kms$^{-1}$ in the nebular-phase spectrum of this SN Ia. High luminosity H$\alpha$emission ($L_{{\rm H}\alpha}\gtrsim 10^{40}$ ergs~s$^{-1}$) has previously beendiscovered in a rare class of SNe Ia-like objects showing CSM interactions (SNeIa-CSM). They predominantly belong to over-luminous ($M_{\rm max}<-19$ mag inoptical) 1991T-like SNe Ia and are exclusively found in star-forming galaxies.By contrast, ASASSN-18tb is a sub-luminous SN Ia ($M_{B, {\rm max}}\sim -17.7$mag) found in an early-type galaxy dominated by old stellar populations. Wediscuss possible origins for the observed hydrogen. Out of 75 SNe Ia for whichwe have so far obtained nebular spectra in 100IAS, no other SN shows a $\sim1000 \,{\rm km s^{-1}}$ H$\alpha$ emission line with comparable line luminosityas ASASSN-18tb, emphasizing the rarity of such emission in the nebular phase.Based on preliminary results from our survey, the rate for ASASSN-18tb-likenebular H$\alpha$ emission could be as high as $\sim 10\%$ level amongsub-luminous SNe Ia.

H-$\alpha$ emission in the nebular spectrum of the Type Ia supernova ASASSN-18tb