2006A&A...451..563I

The spectral evolution of an extremely slow nova V723 Cas was monitored at Asiago Observatory for seven years. Observations in the long pre-maximum stage, which lasted more than four months, were reported in our previous paper. Here, we report the spectral evolution in the decline stage from January 1996 to December 2001. The spectra just after maximum luminosity showed prominent emission lines of H I, He I, Fe II, Ti II, Si II, Na I, etc. P Cygni type absorption components appeared on H I, He I, and Fe II lines about two months after maximum when the nova performed the second brightening. The radial velocities of the absorption components of H I and He I lines varied from -600km/s in February to -1600km/s in June 1996. Broad emission wings of H I and He I lines that were red-shifted by about 1000km/s were detected in the same season. When the third brightening began in July 1996, a new absorption system of H I and He I appeared in which the blue-shift changed from -1000km/s in July 1996 to -1600km/s in February 1997. Prominent emission lines of highly ionized ions such as [Fe VI], [Fe VII], and [Ca V] emerged in the period between May 30 and July 1, 1997. This nova probably entered the nebular stage in this period. The interval from maximum luminosity to the beginning of nebular stage is about 18 months, which is still longer than those of the other extremely slow novae: 10 months of RR Pic or 8 months of HR Del. Some emission lines of [Fe IV], which were not found on the spectra of usual classical novae, were detected on the spectra in the nebular stage. The ionization state grew with time to have a coronal emission line [Fe X] l6375 in November 1999 and later. The interstellar extinction is estimated at E(B-V)=0.57±0.05 and the distance to the nova at about d=2.8kpc, while the mass of the ejecta may be about 5x10^–6M_☉. The abundance of helium is estimated at N(He)/N(H)=0.18± 0.01. The abundance of oxygen is not so different from those of usual classical novae, while a rather low abundance of nitrogen is noticed. The abundance of neon is roughly comparable to that of a neon nova QU Vul.