  Recurrent novae are binary stars in which a white dwarf accretes matter from
a less evolved companion, either a red giant or a main-sequence star. They have
dramatic optical brightenings of around 5-6 mag in V in less than a day,
several times a century. These occur at variable and unpredictable intervals,
and are followed by an optical decline over several weeks, and activity from
the X-ray to the radio. The unpredictability of recurrent novae and related
stellar types can hamper systematic study of their outbursts. Here we analyse
the long-term lightcurve of RS Ophiuchi, a recurrent nova with six confirmed
outbursts, most recently in 2006 February. We confirm the previously suspected
1945 outburst, largely obscured in a seasonal gap. We also find a signal via
wavelet analysis that can be used to predict an incipient outburst up to a few
hundred days before hand. This has never before been possible. In addition this
may suggest that the preferred thermonuclear runaway mechanism for the
outbursts will have to be modified, as no pre-outburst signal is anticipated in
that case. If our result indeed points to gaps in our understanding of how
outbursts are driven, we will need to study such objects carefully to determine
if the white dwarf is growing in mass, an essential factor if these systems are
to become Type Ia Supernovae. Determining the likelihood of recurrent novae
being an important source population will have implications for stellar and
galaxy evolution.
