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\IBVShead{5248}{26 March 2002}

\IBVStitletl{On the Period of the High Amplitude }{ {\large \boldmath $\delta$} Scuti Variable DW P\lowercase{sc}\IBVSfm}

\IBVSfoot{Based on observations made at Beersel Hills Observatory (Belgium) and the 2.0m telescope operated by the Institute of Astronomy (Bulgaria)}

\IBVSauth{Van Cauteren, P.$^{1,2}$; Wils, P.$^2$; Lampens, P.$^3$; Strigachev, A.$^{3,4}$}

\IBVSinst{Beersel Hills Observatory, Laarheidestraat 166, B-1650 Beersel, Belgium,\\ email: Paul.VanCauteren@pi.be}
\IBVSinst{Vereniging Voor Sterrenkunde, Belgium, email: Patrick.Wils@cronos.be}
\IBVSinst{Koninklijke Sterrenwacht van Belgi\"e, B--1180 Brussel, Belgium, 
\\ email: Patricia.Lampens@oma.be}
\IBVSinst{Institute of Astronomy, Bulgarian Academy of Sciences, 72 Tsarigradsko Chausse Blvd., 1784 Sofia, Bulgaria}

\SIMBADobjAlias{DW Psc}{GSC 614-1209}

\IBVSkey{Delta Scuti stars - photometry}

\IBVSabs{CCD photometric data confirm that DW Psc is a}
\IBVSabs{High Amplitude Delta Scuti (HADS) variable with a period of}
\IBVSabs{0.0596489 days. The period may have undergone a sudden change in 1999.}

\begintext

The star DW Psc (=GSC 614-01209; $\alpha_{2000}= 01\hr30\mm26\fsec9$;
$\delta_{2000}= +08\deg41\arcm34\arcs$) was discovered by Krugly (1999) as a new $\delta$ Scuti variable
with a period of 0\fday05875 in the magnitude range 13\fmm7-14\fmm4.

It was observed at Beersel Hills Observatory (BHO) on one night in November 1999 and
six nights between November 2001 and
February 2002.  A total of 735 data points were obtained during 14.7 hours
of photometry. The instrument used was a 0.4m telescope, equipped with a 
ST7E CCD camera. No filter was used. The exposure times varied between 60 and 120 seconds.
In addition 36 datapoints were obtained during a 2 hour-run in November 2001 at the
National Astrophysical Observatory (NAO) at Rohzen, (Bulgaria), with the
Photometrics CE200A CCD camera attached to the 2.0m telescope. The exposures
were taken in V-light with exposure times between 100 and 150 seconds.
All images were reduced with the aperture photometry procedure of the Mira
AP software package\footnote{The Mira AP software is produced by Axiom Research Inc.} and darkframed and flatfielded according to standard procedures.


The brightness of the variable was measured with respect to GSC 614-00524, while GSC 614-00592
was used as a check star.  Both stars have $\Delta (B-V)$= 0\fmm39 with
respect to
the variable. These instrumental values of $\Delta (B-V)$
were determined at BHO from images in B and V light, using a filterset following Bessel's specifications.
The nightly standard deviation of the differences in unfiltered magnitudes between the comparison and
the check star measured at BHO, averaged 0\fmm03.  At Rozhen Observatory this
standard deviation amounted
to 0\fmm009.

The following times of maxima have been determined by fitting a third degree polynomial through the data
around maximum (the $O-C$ values are listed with respect to the ephemeris derived below):

\begin{center}
\begin{tabular}{ccrcc}
\hline
JD Hel. & E & \multicolumn{1}{c}{$O-C$} & Observer & Instrument\\
&& \multicolumn{1}{c}{[$d$]} && \\
\hline
2451498.3283 & $-$12088 & $-$0.0006 & PVC & BHO, 0.4m \\
2451498.3892 & $-$12087 & 0.0005 & PVC & BHO, 0.4m \\
2452219.3644 & 0 & $-$0.0001 & PVC & BHO, 0.4m \\
2452223.3618 & 67 & 0.0008 & PL & NAO, 2.0m \\
2452223.3618 & 67 & 0.0008 & PVC & BHO, 0.4m \\
2452223.4198 & 68 & $-$0.0009 & PL & NAO, 2.0m \\
2452223.4203 & 68 & $-$0.0004 & PVC & BHO, 0.4m \\
2452224.2557 & 82 & 0.0000 & PVC & BHO, 0.4m \\
2452224.3153 & 83 & $-$0.0001 & PVC & BHO, 0.4m \\
2452228.3119 & 150 & 0.0000 & PVC & BHO, 0.4m \\
2452228.3714 & 151 & $-$0.0002 & PVC & BHO, 0.4m \\
2452228.4319 & 152 & 0.0008 & PVC & BHO, 0.4m \\
2452232.3681 & 218 & 0.0001 & PVC & BHO, 0.4m \\
2452308.3007 & 1491 & $-$0.0003 & PVC & BHO, 0.4m \\
2452308.3601 & 1492 & $-$0.0005 & PVC & BHO, 0.4m \\
\hline
\end{tabular}
\end{center}

From these maxima, the following ephemeris was derived:
$$\begin{array}{r@{}r@{}l}
{\rm Max. = HJD}~2452219\fday3645 & {} + 0\fday05964887 & {} \times E. \\
{} \pm 0.0002 & {} \pm 0.00000003 \\
\end{array}$$


\IBVSfig{10cm}{5248-f1.eps}{Phased light curve for DW Psc (BHO, unfiltered magnitudes with respect to GSC 614-00524).}

\newpage

\IBVSfig{10cm}{5248-f2.eps}{Phased light curve for DW Psc (NAO, V magnitudes with respect to GSC 614-00524).}


Fig.~1 shows the phased light curve from the unfiltered BHO data (2001-2002) and Fig.~2 the phased light curve from the V data taken at NAO.

With the same method as above, a maximum was calculated from the data of Krugly (1999), 
giving HJD = 2450698.5135 and
a large $O-C$ = 0\fday0163 (E = $-$25497).  Adding one hour to this time of maximum would bring 
the $O-C$ back to $-$0\fday0017 (E = $-$25496), which lies within the errors of the formula.  
However Krugly (2002, private communication) asserts that his observation
times are correct (by comparing them with the positions of the asteroid 2100 
Ra-Shalom
which is on the images).  If this is the case, the period of DW Psc has undergone a sudden change $\Delta P/P$ 
of at least $-2 \times 10^{-5}$ 
somewhere before the first observations at BHO in 1999.  Thereafter the period seems to be constant.  
Although uncertain in this case, this sudden change is some 10 times larger than what is 
observed in Pop. II $\delta$ Scuti (SX Phe) stars (Breger and Pamyatnykh, 1998).  
However, we recall that an abrupt
period break was detected in the case of V1162 Orionis (Hintz et.al.,
1998).  Intensive monitoring of this star later on showed that period
changes of magnitude $-1.6 \times 10^{-5}$ are occurring (Arentoft et. al., 2001).
Further observations of DW Psc would be needed to verify the constancy of the period or
to disclaim it.

In the light curve of DW Psc at least four harmonics of the fundamental period can be detected.  After prewhitening
there is some evidence for further frequencies in the range 20$-$30 c/d, with an amplitude comparable to the observational error and
a signal-to-noise ratio around 4. However, strong aliasing is involved.
The timespan of the observations is too short to be conclusive about these frequencies.
This is another reason to monitor the star in the future.

\newpage

{\it Acknowledgements:}  Dr. Y.N. Krugly is acknowledged for checking his observation times.  
P. Van Cauteren is grateful to the Royal Observatory of Belgium for putting at his disposal material
acquired by project G.0265.97 of the Fund for Scientific Research (FWO) - Flanders (Belgium).
P. Lampens and A. Strigachev acknowledge support received from the
Belgian Federal Office for Scientific, Federal and Cultural Affairs (OSTC) and from
the Bulgarian Academy of Sciences (BAS) in the framework of the project B9 ``Astrometric,
photometric and spectroscopic follow-up of binaries''.


\references

Arentoft, T., Sterken, C., Knudsen, M.R., Handler, G., Niarchos, P., Gazeas, K., Manimanis, V., Moalusi, M.B., Vuthela, F.F. and Van Cauteren, P., 2001, {\it A\&A}, {\bf 378}, L33

Breger, M. and Pamyatnykh, A.A., 1998, {\it A\&A}, {\bf 332}, 958

Hintz, E.G., Joner, M.D., Kim, C., 1998, {\it PASP}, {\bf 110}, 689

Krugly, Y.N., 1999, {\it IBVS}, No.\ 4730

\endreferences


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