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\IBVShead{4852}{16 February 2000}

\IBVStitle{Orbital Period and Oscillations in V723 Cassiopeiae}

\IBVSauth{Goranskij, Vitalij P.$^1$; Shugarov, Sergei Y.$^1$; Katysheva, Nataly A.$^1$;\\ \indent Shemmer, Ohad$^2$; Retter, Alon$^{2,3}$; Chochol, Drahom\'{i}r$^4$; Pribulla, Theodor$^4$}

\IBVSinst{Sternberg Astronomical Institute, Universitetskii prospect,
13, Moscow, 119899, Russia}

\IBVSinst{School of Physics and Astronomy and the Wise Observatory,
Tel-Aviv University, Tel Aviv, 69978, Israel}

\IBVSinst{Physics Department, Keele University, Staffordshire, ST5 5BG,
United Kingdom}

\IBVSinst{Astronomical Institute of the Slovak Academy of Sciences, 059
60 Tatransk\'a Lomnica, Slovakia,\\ \indent email: chochol@auriga.ta3.sk}

\SIMBADobjAlias{V723 Cas}{Nova Cas 1995}

\begintext

V723 Cas (Nova Cas 1995) was discovered by M.~Yamamoto on August 24,
1995 (Hirosawa, 1995) and reached the maximum in $V = 7\fmm09$, $B =
7\fmm59$ and $R = 6\fmm59$ on December 17, 1995 in a short-lived
flare.  The faint star of $R = 17\fmm5$ and $B = 19\fmm0$ has been
identified as the precursor of the nova (Goranskij et al., 1997). An
overall decline from the maximum was accompanied by large flares in
1996. Since 1997 the continuous brightness decline was interrupted only
by small flares (at JD 2450670 and 2450860) and variability on
different time scales. The brightness of the nova declined to $V =
13\fmm66$, $B = 14\fmm29$ and $U = 13\fmm41$ on January 12, 2000.

The photometric observations taken before December 1996 did not show
any strict short-term periodicity. The period analysis of the UBV
photoelectric observations taken during quiescent stages (JD
2450421--468, 2450704--794) revealed two possible photometric periods:
0.6350 and 0.6818 days (Chochol \& Pribulla, 1998). The folded light
curve supported the former one.

The present study is based upon an extensive photometric monitoring at
the Sternberg Astronomical Institute in Moscow (SAI), the Tel-Aviv
University Wise Observatory (WO) and the Astronomical Institute of the
Slovak Academy of Sciences (AISAS).

The SAI monitoring consists of 1203 $R$ band frames taken with the SBIG
ST-7 CCD during 19 nights from September to December, 1999 (JD
2451432--2451538). The 60-cm reflector of the Sternberg Institute
Crimean station, 38-cm reflector of the Crimean Astrophysical
Observatory, and 30-cm refractor of the Sternberg Institute in Moscow
were used. The accuracy of CCD photometry varies in the range of
0\fmm005--0\fmm02 in the $R$ bandpass. Additionally a few night sets of
CCD and photoelectric photometry in $B$, $V$ and $I$ bandpasses were
obtained during the same time interval.

The WO monitoring performed by a 1-m telescope consists of 1913 $R$
band frames taken with the Tektronix 1K CCD camera (described by Kaspi
et al., 1995) during 23 nights from October 1996 to September 1998 (JD
2450378--2451064). The CCD photometry was performed also in $U$, $B$,
$V$ and $I$ bandpasses.

The $UBVR$ photoelectric photometry at the AISAS using two 60-cm
reflectors of the Star\'a Lesn\'a and Skalnat\'e Pleso observatories
was performed on 152 nights from August 1995 to January 2000
(2449961--2451556). Part of the observations (till March 1998) was
already published and analysed (Chochol \& Pribulla, 1997, 1998).

\IBVSfig{9.5cm}{4852-f1.ps}{1996 to 1999 evolution of the $R$ folded
light curve of Nova Cas 1995 obtained using the ephemeris (1). The
1996--1998 $R$ data are in instrumental system. Note the different vertical scales for the respective panels}

\vskip-11pt
The photometric period search was accomplished using the AISAS $V$
observations during the quiescent stages (JD 2450421--468,
2450704--794) combined with the WO and the SAI $R$ observations
obtained after JD 2450437. To avoid  the problems with the brightness
decline and variability on a time scale of tens of days, only the longer
runs or subsequent night's data were used. To ascertain the photometric
period, the Fourier period analysis  was applied. The most significant
period in the range 0.1--1 day is 0\fday69325 (Chochol et al., 2000)
close to the period 0\fday6818 suggested by Chochol \& Pribulla (1998).
The ephemeris for the brightness minima is
\begin{equation}
\begin{array}{r@{}r@{}l}
{\rm Min~(I) = HJD}~2\,450\,421.4801 & {} + 0\fday69325 & {} \times E. \\
                            {} \pm 7 &        {} \pm 18 \\
\end{array}
\end{equation}

The evolution of the $R$ band folded light curves in the years
1996--1999 is shown in Fig.~1. The dispersion of the light curves
reflects the decay of the mean light combined with the long-term
variability. The observations taken in October and November 1996 show
only quasi-periodic variations.  The periodicity of the brightness
changes started to be visible in December 1996. During the 1997
observation season the photometric period displayed itself as a
continuous wave-like variation. In 1998 the amplitude of the variation
increased and the light curve changed to a saw-like shape. The full
amplitude of the $R$ variations increased from 0\fmm13 in 1998 to
0\fmm35 in 1999.

We have detected 5 minima times in $B$ and $R$ bands at JD
2451\dots\ 438.53, 441.27, 445.41, 467.64 and 538.33 during the 1999
observation season. The repeating dip in the phase of 0.6, seen in the
1999 data, may be an eclipse.

Besides, the apparent periodic oscillations were occasionally seen
through the 1997--1999 observation seasons. For instance, three
consecutive humps are well seen on the 1999 light curve (Fig.~1) near
light maximum in the night of JD 2451441 with local maxima at
0\fday462, 0\fday520 and 0\fday587 which suggest the period of
0\fday062. The amplitude of oscillation during this night reached
0\fmm05. Other possible maxima of the humps are seen in the different
orbital phases at JD 2451\dots\ 432.482, 432.533, 438.573, 485.500,
485.565, 492.517, which improves the period of the oscillation to
$0\fday061512 \pm 0\fday000003$. The majority of night sets do not show
the oscillation and period analysis of the total set did not reveal the
corresponding frequency. During the 1998 observation season we detected
only two humps on September 6 at JD 2451\dots\ 063.34 and 063.46 giving
a twice longer period than in 1999. Several solitary humps were also
recorded in 1997 (e.g.\ 2450706.37).

We have verified the constancy of the comparison star with two check
stars. Therefore the oscillations have to be attributed to the nova.

Munari et al.\ (1996) pointed out that V723 Cas displays some
resemblance to the classical novae like HR Del but also to symbiotic
novae. Our photometry shows that V723 Cas is a cataclysmic binary
system with a photometric period of 0\fday69325, which we interpret as
its orbital period.

\bigskip

{\it Acknowledgements.} This study was financially supported by the
Scientific and Technical program ``Astronomy'' of Russian Ministry of
Science (V.G., S.S.), grants of Russian Foundation of Basic Researches
96--02--19017 (S.Sh.) and 96--15--96489  (N.K., S.Sh.), Russian
Universities grant N\,5558 (N.K., S.Sh.), VEGA grant 5038/2000 (D.C.,
T.P.) and PPARC (A.R.). Astronomy at the WO is supported by grants from
the Israeli Academy of Sciences.

\references

Chochol, D., Pribulla, T., 1997, {\it Contrib.\ Astron.\ Obs.\ Skalnat\'e
Pleso}, {\bf 27}, 53

Chochol, D., Pribulla, T., 1998, {\it Contrib.\ Astron.\ Obs.\ Skalnat\'e
Pleso}, {\bf 28}, 121

Chochol, D., Pribulla, T., Shemmer, O., Retter, A., Shugarov, S.Y.,
Goranskij, V.P., Katysheva, N.A., 2000, {\it IAU Circ.}, No.\ 7351

Goranskij, V.P., Karitskaya, E.A., Kusakin, A.V., et al., 1998, {\it
Astron.\ Astrophys.\ Trans.}, {\bf 15}, 101

Hirosawa, K., 1995, {\it IAU Circ.}, No.\ 6213

Kaspi S., Ibbetson, P.A., Mashal E., Brosch N., 1995, {\it Wise Obs.\
Tech.\ Report}, No.\ 6

Munari, U., Goranskij, V.P., Popova, A.A., et al., 1996, {\it Astron.\
Astrophys.}, {\bf 315}, 166

\endreferences

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