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 (lambda/Delta lambda approximate to 7000-10 000) hydrogen and iron profiles suggest an early expansion velocity of 2000-2500 km s(-1). The H alpha evolution is followed from Deltat = -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.(m)85 +/- 0.(m)3. The available visual and CCD-V observations are used to determine t(0), t(2) and t(3). The resulting parameters are: t(0) = 2451669.5 +/- 0.1, t(2) = 30 +/- 1 d, t(3) = 36 +/- 1 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.