[en] If accreting white dwarfs (WDs) in binary systems are to produce type Ia supernovae (SNe Ia), they must grow to nearly the Chandrasekhar mass and ignite carbon burning. Proving conclusively that a WD has grown substantially since its birth is a challenging task. Slow accretion of hydrogen inevitably leads to the erosion, rather than the growth of WDs. Rapid hydrogen accretion does lead to growth of a helium layer, due to both decreased degeneracy and the inhibition of mixing of the accreted hydrogen with the underlying WD. However, until recently, simulations of helium-accreting WDs all claimed to show the explosive ejection of a helium envelope once it exceeded $\sim <!--\; ???\; -->{10}^{-<!--\; ???\; -->1}{M}_{\odot <!--\; ???\; -->}$. Because CO WDs cannot be born with masses in excess of $\sim <!--\; ???\; -->1.1M_{\odot <!--\; ???\; -->}$, any such object in excess of $\sim <!--\; ???\; -->1.2M_{\odot <!--\; ???\; -->}$ must have grown substantially. We demonstrate that the WD in the symbiotic nova RS Oph is in the mass range 1.2–1.4 M _☉. We compare UV spectra of RS Oph with those of novae with ONe WDs and with novae erupting on CO WDs. The RS Oph WD is clearly made of CO, demonstrating that it has grown substantially since birth. It is a prime candidate to eventually produce an SN Ia.