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\IBVShead{5968}{28 January 2011}

\IBVStitletl{Periodicities of a Nova-Like}{ Cataclysmic Variable Star RX\,J1951.7+3716}

\IBVSauth{Zubareva, A. M.$^1$; Antipin, S. V.$^{2,1}$}

\IBVSinst{Institute of Astronomy, Russian Academy of Sciences,
48, Pyatnitskaya Str., 119017 Moscow, Russia}

\IBVSinst{Sternberg Astronomical Institute, 13, University Ave.,
119992 Moscow, Russia}

\IBVSinsto{email: zubareva@inasan.ru, antipin@sai.msu.ru}



\SIMBADobj{RX J1951.7+3716}

\IBVStyp{  NL  }

\IBVSkey{stars: Cataclysmic variables }

\IBVSabs{We confirm variability of RX\,J1951.7+3716 based on our}
\IBVSabs{CCD observations. The results of search for}
\IBVSabs{periodicities are presented.}


\begintext

Motch et al. (1998) classified RX\,J1951.7+3716 ($\alpha$ =
19\hr51\mm47\fsec50, $\delta$ = +37\deg16\arcm47\farcs8; J2000.0)
as a cataclysmic variable based on the X-ray and optical
characteristics of the object. Peters and Thorstensen (2005)
obtained optical spectra of the star and analyzed radial velocity
changes using both absorption and emission lines. The authors
found an orbital period of the system, $P_{orb}=0\fday492(1)$.
There were no studies on brightness variations of RX\,J1951.7+3716
published to date.

\IBVSfig{6cm}{5968-f1.eps}{The summary light curve.}
\IBVSfigKey{5968-f1.eps}{RX J1951.7+3716}{light curve}

To confirm or disclaim present classification we have investigated
RX\,J1951.7+3716 photometrically. Our observations were carried
out at the 60-cm telescopes of the Terskol Branch of the Institute
of Astronomy and the Crimean Laboratory of Sternberg Astronomical
Institute equipped with a PixelVision SpectraVideoTM and an Apogee
AP-47p CCD cameras respectively. We monitored RX\,J1951.7+3716
for eighteen nights on June~10--August~23, 2010 (JD~2455358--2455432).
 The frames were taken with 180~s (Terskol) and
120~s (Crimea) exposure times in the Johnson $R$ filter. 2347
images were debiased, dark-subtracted, flat-fielded and then
analyzed in MaxIm DL 4 package. \hbox{USNO-A2.0~1200-13535122}
($\alpha$~=~19\hr51\mm50\fsec37,
$\delta$~=~+37\deg15\arcm56\farcs0; J2000.0; photographic $R$~magnitudes: 
$13\fmm8$ in the USNO-A2.0 and $13\fmm98$, $13\fmm83$
in the USNO-B1.0 catalog) was used for comparison. Stability of
the comparison star was verified by brightness measurements with
respect of several check stars. The uncertainty of our photometry
is about $0\fmm03$. The summary light curve is shown in Fig.\,1.

The amplitude of light variations of RX\,J1951.7+3716 during
different nights of our set changes from $0\fmm4$ to $0\fmm8$. The
shape of the light curves is composite. The most stable feature of
brightness variations is a flickering on timescales of 15--25~minutes. 
Some of light curves contain steep brightness rises and
declines (Fig.\,2). This photometric behavior is typical of
nova-like cataclysmic variables (Warner, 1995).

\IBVSfig{11cm}{5968-f2.eps}{Individual light curves for several
nights of observations.}
\IBVSfigKey{5968-f2.eps}{RX J1951.7+3716}{light curve}

\vspace*{-0.7cm}

\IBVSedata{5968-f1.txt}
\IBVSdataKey{5968-t1.txt}{RX J1951.7+3716}{photometry}

We have analyzed our data (available through the IBVS website as 
{\tt 5968-t1.txt}) for periodicity using the Lafler and
Kinman method (Lafler and Kinman, 1965) implemented in Pelt's
(Pelt, 1980) and V.P.~Goranskij's packages. The spectral orbital
period (Peters and Thorstensen, 2005) is not present in our
observations explicitly. Looking for periodicity in a very narrow
interval in the vicinity of this period, we find only a low peak
at $0.4928$ days. We hope that the corresponding phased light
curve (Fig.\,3) can be considered as photometric reflection of
the orbital motion. Two features are present in the figure.
First, the wide minimum close to the phase 0.0 is better seen in
the averaged light curve. Second, the flickering at the phases
0.2--0.35 is less prominent than at other intervals. We suppose
that the latter finding can be explained by an eclipse of the
interaction region of the disk with the accretion stream. Notice
that, statistically, cataclysmic variables with orbital periods
longer than ten hours are quite rare (e.g. Warner, 1995).

\IBVSfig{6.5cm}{5968-f3.eps}{The phased light curve for the orbital
period.}
\IBVSfigKey{5968-f3.eps}{RX J1951.7+3716}{phased light curve}

In absence of an evident orbital period, the strongest peaks on
the periodogram correspond to $0\fday628$ and its daily alias
$0\fday3879$ (Fig.\,4, Fig.\,5). We emphasize that these periods
are not coupled with the orbital one. Nature of these
periodicities remains unknown. The flickering being the most
prominent feature of the variable's light curve, a whitening of
the mentioned phased curves for the orbital variability and, vice
versa, a whitening of the orbital curve for 0.628 or 0.3879~day
periodicities does not improve the phased curves.


\IBVSfig{6.5cm}{5968-f4.eps}{The power spectrum constructed using the
Crimean set of observations (2232 images, July~30--August~23,
2010).}
\IBVSfigKey{5968-f4.eps}{RX J1951.7+3716}{periodogram}

\IBVSfig{6.5cm}{5968-f5.eps}{The phased light curve for possible
periodicities.}
\IBVSfigKey{5968-f5.eps}{RX J1951.7+3716}{phased light curve}

\pagebreak

{\bf Acknowledgements:} We thank N.N. Samus for useful
discussion. We are grateful to the anonymous referee for
constructive comments. The study was supported in part by the
Russian Foundation for Basic Research.


\references


Lafler, J. and Kinman, T.D., 1965, {\it ApJ Suppl}, {\bf 11}, 216

Motch, C., Guillout, P., Haberl, F.  et al., 1998, {\it A\&A Suppl}, {\bf 132}, 341

Pelt, J., 1980, {\it Frequency analysis of astronomical time
series}, Tallin, Valgus (in Russian) 

Peters, C.S. and Thorstensen, J.R., 2005, {\it PASP}, {\bf 117}, 1386

Warner, B., 1995, {\it Cataclysmic Variable Stars}, Cambridge,
Cambridge Univ. Press

\endreferences

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