  If accreting white dwarfs (WD) in binary systems are to produce type Ia
supernovae (SNIa), 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 $\sim 10^{-1}\, \rm
M_{\odot}$. Because CO WDs cannot be born with masses in excess of $\sim 1.1\,
\rm M_{\odot}$, any such object, in excess of $\sim 1.2\, \rm M_{\odot}$, 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$_{\odot}$. 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 SNIa.
