  A directed search for previously-undiscovered nova eruptions was conducted in
the astronomical plate archives at Harvard College Observatory and Sonneberg
Observatory. We found that an eruption of V2487 Oph (Nova Oph 1998) occurred on
1900 June 20. V2487 Oph was previously classified as a classical nova, which we
identified as a probable recurrent nova based on its large expansion velocities
and the presence of high excitation lines in the outburst spectrum. The event
was recorded on Harvard plate AM 505, at a B magnitude of 10.27 +/- 0.11, which
is near peak. The outburst can only be seen on one plate, but the image has a
characteristic dumbbell shape (caused by a double exposure) that is identical
to the other star images on the plate, and thus is not a plate defect. We
conclude that this is in fact a previously-undiscovered nova outburst of V2487
Oph, confirming our prediction that it is a recurrent nova. We also examine the
discovery efficiency for eruptions of the system and conclude that a
randomly-timed outburst has, on average, a 30% chance of being discovered in
the past century. Using this, we deduce a recurrence time for V2487 Oph of
approximately 18 years, which implies that the next eruption is expected around
2016.
