1 Introduction

Nova Cassiopeiae 1993, which later received the denotation V705 Cas, was discovered by Japanese amateur astronomer K. Kanatsu on Dec. 7, 1993 as an object not far beyond the naked-eye visibility threshold (of about 6.5 photographic magnitude on the discovery day), at the following coordinates: $\alpha= 23^{\rm h}39^{\rm m}22.3^{\rm s}, \delta= +57^{\circ}14{'}22.55{''}$, eq. 1950.0 (Kanatsu 1993). The nova has reached the brightness maximum ten days after its discovery on Dec. 17, 1993, i.e., on JD = 2 449 339.29, with maximal values of the magnitude in the V and B colours of ($V_{\rm max}= 5.53$ mag and $B_{\rm max}= 6.14$ mag, respectively (Munari et al. 1994b). Its progenitor has been identified on the POSS as a $B \approx 18$ mag star (Skiff 1993). The further development of the nova's light curve morphology has clearly demonstrated it belongs to the DQ Her subclass of novae.

The first optical spectrogram of this nova was obtained by Schmidt (1993), already four days before the maximum. The nova appeared to be a Fe-II class object (Williams 1992), with an emission spectrum including many Fe II, O I, Ca II as well as H$_{\alpha}-{\rm H}_{\gamma}$ lines. Woodward & Greenhouse (1993) reported the spectrum obtained on Dec. 15 with FWHM from Balmer lines of 1680 km^-1. Shore et al. (1994b) concluded from the IUE observations that the optical luminosity of this nova was larger than its UV flux by about a factor of 10.

The resemblance of V705 Cas to nova DQ Her was further strengthened by its spectral behaviour because its spectra obtained soon after the outburst maximum showed optically thick Fe II lines (Starrfield et al. 1995). These authors took the UV spectra of the nova (using the IUE satellite) just a few hours after the announcement of the discovery. Comparing these early spectra with a later one obtained on Jan. 28, 1994, they showed that the effective temperature of the shell of the nova had increased from 8 000 K (on Dec. 13 1993) to 24 000 K until the day they obtained the later spectrum, whereas its expansion velocity had decreased from 2000 km^-1 to 1300 km^-1 over the same interval. The identical results were presented by Hauschildt et al. (1994), who concluded that the radiation pressure alone was sufficient to attain the observed high expansion velocity of the shell. Besides that, on the basis of the interstellar lines of the doublet Na I D a Mg II, they have estimated the distance to the nova to be between 3 and 6 kpc - which is, as they noted, inconsistent with the distance value (about 2 kpc) determined from the light curve. On the other hand, the comparison of the nova's properties with those of DQ Her yields the lower limit for the distance determination of 3.4 kpc.

Elkin (1995) presented the results of spectroscopic observations of V705 Cas with the 6-m telescope at the SAO. The spectrum of the nova exhibited a large number of emission and absorption features and two easily discernible radial velocity systems. The primary absorption spectrum displayed radial velocity of -550 km^-1 and the diffuse-enhanced one that of -1330 km^-1. Gehrz et al. (1995) have published infrared spectrophotometric and photometric observations of the dust shell of V705 Cas in the interval covering days 130 to 418 after the outburst. Their measurements suggest that the dust shell was optically thick at least until 131 days after the outburst, later it exhibited features of an optically thin shell composed of silicon grains and of some hydrocarbons. On Jul. 22, 1995, the expanding shell of the nova was also detected in the radio part of the spectrum by Eyres et al. (1996), while they were observing with the MERLIN array at the wavelength of 6 cm. The angular dimension of the shell was then approximately 0.2 arcsec, which corresponded to the distance of 2.5 kpc. Shore et al. (1994a) have studied the composition of the dust shell. On the basis of the extinction curve, they concluded that the nature of the dust formed in the vicinity of the nova is strictly different from that of the typical interstellar material. Their observations can be interpreted by the existence of large silicon grains within the dust shell, assuming the simultaneous presence of the stellar wind.

Evans et al. (1996a,b) obtained the IR spectroscopic data for this nova and detected conspicuous bands of CO which have been present already during its brightness maximum. To our knowledge, the most recent paper published on V705 Cas presents the results of the IR spectroscopy obtained on day 367 after the maximum (Lynch et al. 1997). These authors mention the strong IR excess and deduce the shell temperature to be within the interval from 710 K to 320 K, depending on the specific wavelength.

The light behaviour of V705 Cas in the prenova stage was investigated by Munari et al. (1994a) on the archival photographic plates of the Asiago Astrophysical Observatory collection. No significant brightness fluctuations of the progenitor were found. Hauschildt et al. (1994) have described the morphological features of the light curve of V705 Cas and pointed out its close resemblance to DQ Her. Both novae exhibited premaximum halt, then rapid increase to maximum, an eruption of several magnitudes shortly after the beginning of the maximum plateau stage and slow early decline = by 2-3 mag over 2-3 months before they entered the deep decline of the transition stage. Retter & Leibowitz (1995) performed CCD photometric observations of V705 Cas in the I colour during 10 nights in Aug. and Sep. 1995. The period analysis of these data yielded the detection of light variations of this nova with the most probable period of 0.228 days and the amplitude of 0.05 mag. Boyle (1994), along with his collaborators, has analyzed the direct CCD images of the nova shell and detected an asymmetric emission shell with a radius of about 9 arcsec in the vicinity of the nova, which by far exceeds any reasonably expected dimensions of the presently ejected shell, given its observed expansion velocity. Therefore, he interprets the detected structure as a relic of the previous outburst of the nova which could have occurred some time between 1920 and 1960.

Several authors have attempted to deduce the basic physical parameters of this nova preferentially on the basis of its brightness decline. Chochol et al. (1995) determined the parameters of the nova from the photometric observations obtained during the first 2 months which have elapsed after the light maximum. Their derived distance value was 2.36 kpc. Elkin (1995) has deduced the nova's distance using several independent methods. As for polarimetric observations, he presented the value of 940 pc. From the interstellar Ca II K line, he derived the distance of 880 pc. Finally, on the basis of light curve analysis, he attained the value of 1350 pc. Lynch et al. (1997) have determined the distance to the nova to be 1270 pc on the basis of the visual observations obtained by the AAVSO members and of those communicated via VSNET, with a probable error of $\pm 370$ pc.