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The recurrent nova M31N 2008-12a experiences annual eruptions, contains anear-Chandrasekhar mass white dwarf, and has the largest mass accretion rate inany nova system. In this paper, we present Hubble Space Telescope (HST)WFC3/UVIS photometry of the late decline of the 2015 eruption. We couple thesenew data with archival HST observations of the quiescent system and Keckspectroscopy of the 2014 eruption. The late-time photometry reveals a rapiddecline to a minimum luminosity state, before a possible recovery /re-brightening in the run-up to the next eruption. Comparison with accretiondisk models supports the survival of the accretion disk during the eruptions,and uncovers a quiescent disk mass accretion rate of the order of$10^{-6}\,M_\odot\,\mathrm{yr}^{-1}$, which may rise beyond$10^{-5}\,M_\odot\,\mathrm{yr}^{-1}$ during the super-soft source phase - bothof which could be problematic for a number of well-established nova eruptionmodels. Such large accretion rates, close to the Eddington limit, might beexpected to be accompanied by additional mass loss from the disk through a windand even collimated outflows. The archival HST observations, combined with thedisk modeling, provide the first constraints on the mass donor;$L_\mathrm{donor}=103^{+12}_{-11}\,L_\odot$,$R_\mathrm{donor}=14.14^{+0.46}_{-0.47}\,R_\odot$, and $T_\mathrm{eff,donor}=4890\pm110$ K, which may be consistent with an irradiated M31 red-clumpstar. Such a donor would require a system orbital period $\gtrsim5$ days. Ourupdated analysis predicts that the M31N 2008-12a WD could reach theChandrasekhar mass in < 20 kyr.

Inflows, Outflows, and a Giant Donor in the Remarkable Recurrent Nova M31N 2008-12a? - Hubble Space Telescope Photometry of the 2015 Eruption