Near-infrared and optical studies of the fast nova V4643 Sgr (Nova Sagittarii 2001)

V4643 Sagittarii or Nova Sagittarii 2001 was discovered in outburst at 7.7 mag 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 H I Brackett series and also the Paschen β and γ lines. We study the cause of the excitation of the O I line at 1.128 μm 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 O I 1.128-μm line behaviour. An optical spectrum is presented which shows that He II lines are the most prominent features ‐ after H I ‐ 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 [Si VI] and [Si VII] at 1.9641 and 2.4807 μm, respectively.