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If recurrent novae are progenitors of Type Ia supernovae, their white dwarfsmust have masses close to the Chandrasekhar limit. The most reliable means ofdetermining white dwarf masses in recurrent novae is dynamically, viaradial-velocity and rotational-broadening measurements of the companion star.Such measurements require the system to be both eclipsing and to showabsorption features from the secondary star. Prior to the work reported here,the only dynamical mass estimate of a recurrent nova was for U Sco, which has awhite dwarf mass of 1.55 +/- 0.24 Msolar (Thoroughgood et al. 2001).  We present new time-resolved, intermediate-resolution spectroscopy of theeclipsing recurrent nova CI Aquilae (CI Aql) during quiescence. We find themass of the white dwarf to be 1.00 +/- 0.14 Msolar and the mass of thesecondary star to be 2.32 +/- 0.19 Msolar. We estimate the radius of thesecondary to be 2.07 +/- 0.06 Rsolar, implying that it is a slightly-evolvedearly A-type star. The high mass ratio of q = 2.35 +/- 0.24 and the highsecondary-star mass implies that the mass transfer occurs on a thermaltimescale. We suggest that CI Aql is rapidly evolving into a supersoft X-raysource, and ultimately may explode as a Type Ia supernova within 10 Myr.

CI Aql: a Type Ia supernova progenitor?