2006MNRAS.368..592A

V4643 Sagittarii or Nova Sagittarii 2001 was discovered in outburst at 7.7mag on 2001 February 24. Here, we present near-infrared results of this fast classical nova obtained in the early decline phase in 2001 March followed by optical observations about one month later. Subsequently, we also present near-infrared spectra taken later in the nova's evolution, about four months after the outburst, when V4643 Sgr had entered the coronal phase. The spectra in the early decline phase are dominated by emission lines of the Hi Brackett series and also the Paschen β and γ lines. We study the cause of the excitation of the Oi line at 1.128µm and discuss the variation in its strength with time after outburst. We discuss the role of optical depth effects on the observed strengths of the hydrogen Brackett and Paschen lines and discuss possible reasons for the puzzling behaviour of the Brγ line strength and whether it is correlated with the Oi1.128-µm line behaviour. An optical spectrum is presented which shows that Heii lines are the most prominent features - after Hi- to be seen in early 2001 April. We present and also discuss spectra taken in 2001 June and August which prominently show coronal lines of [Sivi] and [Sivii] at 1.9641 and 2.4807µm, respectively.