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 \pm 0.11 , which is near peak .
The outburst can only be seen on one plate , but the image has a characteristic ÔdumbbellÕ 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 .