Detailed optical behavior of the 2000 outburst of the eclipsing recurrent nova CI Aquilae

The 2000 outburst of the recurrent nova CI Aql was followed by optical photometry and spectroscopy. Our time-resolved photometry revealed its intraday variations during the outburst. The orbital modulation of the light curve appeared after entering the plateau stage. We found that primary eclipses were ~0.6 mag in depth, but the profile of the eclipse was significantly different from that in the quiescent phase. The folded orbital light curve was represented by a wide wing of a primary eclipse and it suggests the existence of the accretion disk at the plateau stage. In this outburst, we obtained accurate determination of several minima of primary eclipses, and found that the timings of minima showed a substantial delay compared to the previously reported ephemeris. However, no significant evidence of a change in the orbital period was observed since the discovery of the eclipsing nature of this object. We examined the evolution of optical spectra through the outburst, which reconfirmed the nova nature of this object. A spectrum taken on 2000 October 10 showed the Hαin emission and indicated that the object had not yet reached quiescence. The overall light curve and late-stage spectroscopy have revealed that the plateau is the longest one among recurrent novae.