The 2000 outburst of the recurrent nova CI Aquilae: optical spectroscopy

We present low- and medium resolution spectra of the recurrent nova CI Aquilae taken at 14 epochs in May and June, 2000. The overall appearance is similar to other U Sco-type recurrent novae (U Sco, V394 CrA). Medium resolution (R=7000-10000) hydrogen and iron profiles suggest an early expansion velocity of 2000-2500 km/s. The H\alpha evolution is followed from Dt = -0.6 d to +53 d, starting from a nearly Gaussian shape to a double peaked profile through strong P-Cyg profiles. The interstellar component of the sodium D line and two diffuse interstellar bands put constraints on the interstellar reddening which is estimated to be E(B-V)=0.85\pm0.3. The available visual and CCD-V observations are used to determine t0,t2 and t3. The resulting parameters are: t0=2451669.5\pm0.1, t2=30\pm1 d, t3=36\pm1 d. The recent lightcurve is found to be generally similar to that observed in 1917 with departures as large as 1-2 mag in certain phases. This behaviour is also typical for the U Sco subclass.