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\IBVShead{5377}{17 February 2003}

\IBVStitle{The Period of V2109 C\lowercase{yg} Revisited}

\IBVSauth{G\'omez-Forrellad, J. M.$^{1,2}$}

\IBVSinst{Grup d'Estudis Astron\`omics, Apartado 9481, 08080 Barcelona, Spain; e-mail: jmgomez@astrogea.org}
\IBVSinst{Esteve Duran Observatory Foundation, Avda. Montseny 46, El Montany\`a, 08553 Seva, Barcelona, Spain.}

\SIMBADobjAliasJ2000{V2109 Cyg}{HD 191635}{20 08 50.42 +48 54 39.4}
\IBVStyp{  DSCT  }
\IBVSkey{photometry - delta Sct stars}

\IBVSabs{New observations of V2109 Cyg are used to determine the true}
\IBVSabs{pulsation period of this star. This observations are also}
\IBVSabs{consistent with no period changes since the Hipparcos era.}


\begintext

V2109 Cyg is a variable star discovered by the Hipparcos mission (ESA 1997), and classified as an RRc with a maximum magnitude of 7.49 and variation amplitude of 0.16 mag in the $V$ band. The following ephemeris was also determined:

\begin{center}
${\rm Max.} = {\rm BJD~} 2448500.0280(1) + 0\fday1860656(3)\!\times\!{\rm E}$ \\
\end{center}

Nevertheless, the real physical nature of this star, based only in the morphology of its light-curve and its period, is controversial. Its period, which falls outside the typical observed range for RRc variables, and its light curve morphology, are similar to those displayed
by the largest amplitude $\delta$ Sct stars. Thus, Kazarovets \textit{et al.} (1999) classified V2109 Cyg as a $\delta$ Sct variable, and Rodr\'\i  guez \textit{et al.} (2000) included this object in their catalogue of $\delta$ Sct variables according to photometric data published by Hauck and Mermilliod (1998), who in turn extracted them from the work by Olsen (1983). In addition to this, Kiss \textit{et al.} (1999) observed V2109 Cyg photometrically and spectroscopically, and indicated that it is a monoperiodic RRc star which probably pulsates in the second overtone mode. More recent photometric and spectroscopic observations indicate that this star is, again, a $\delta$ Sct variable (Rodr\'\i  guez 2002).

In their work, Kiss \textit{et al.} also performed a period analysis and found 
for this object a slightly shorter period ($0\fday186049(5)$) than the one 
detected by the Hipparcos mission. They concluded that a sudden period change 
had taken place between 1991 and 1998. However, V2109 Cyg had been observed in 
the $V$ band for six nights a year before, between
16 July and 5 September 1997 from Mollet Observatory (unpublished data), by 
using an automatic 8-cm telescope. After merging Mollet Observatory data with 
the satellite photometry the results obtained by that time matched the period 
obtained by the Hipparcos mission. As a consequence, if there were a period change, it had to happen in the one-year interval between 1997 and 1998. To check this possibility, V2109 Cyg was observed again from Mollet Observatory with a 10-cm telescope for 7 nights, between 1~and~19~July~2001.

The new observations were consistent with those taken in 1997,
ruling out any period shortening. Figure 1 plots V2109 Cyg
photometric data obtained by the Hipparcos satellite and from
Mollet Observatory in 1997 and 2001 folded with the $0\fday186049$ period. 
It can be seen that the phase light-curve cannot
be adequately reconstructed. 

\IBVSfig{7.5cm}{5377-f1.eps}{Light curve of V2109 Cyg folded on the
0\fday186049 period.}

In order to search for a more consistent period, an analysis of the $O-C$ 
residuals based on maximum timings was performed. In this analysis those 
maximum timings obtained by Kiss \textit{et al.} were also included. 
Figure~2a is a plot of $O-C$ residuals according to the Hipparcos ephemeris, 
where it can be seen that the Hipparcos period is slightly shorter than the 
real one as displays the trend of increasingly positive residuals. The new 
period was computed to correct the trend shown in Figure~2a by assuming a 
constant period throughout the entire 1991-2001 interval, and therefore a 
linear increase of $O-C$ residuals based on the satellite ephemeris. For such a
 purpose, a least-squares linear fit was performed on the $O-C$ data. The 
corrected period is $0\fday18606637(22)$, and Figure~2b shows the resulting 
$O-C$ diagram after using the new value. Figure~3 shows the phase curve for 
V2109~Cyg after folding the data according to the 0.18606637-day period. 
This time the phase curve could be satisfactorily reconstructed.

\IBVSfig{7.5cm}{5377-f2.eps}{}

In conclusion, the data seem to be consistent with a constant period in the 
1991-2001 interval. Table~1 gives, as a summary, a list of all known timings 
and $O-C$ residuals after using the $0\fday18606637(22)$ period.

\IBVSfig{7.5cm}{5377-f3.eps}{Light-curve of V2109 Cyg folded on the 0\fday18606637 period.}

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\centerline{Table 1}
\vskip 6pt
\begin{tabular}{cccc} \hline
Maximum & Epoch & $O-C$ & Source \\ \hline
2450673.4677 &   11681 &   $-$0.0026	& present paper \\
2450689.4691 &   11767 &   $-$0.0019	& present paper \\
2450695.6135 &   11800 &   +0.0023	& present paper \\
2450697.4666 &   11810 &   $-$0.0052	& present paper \\
2451032.3936 &   13610 &   +0.0023	& Kiss \textit{et al.} (1999) \\
2451037.4134 &   13637 &   $-$0.0017	& Kiss \textit{et al.} (1999) \\
2451080.3928 &   13868 &   $-$0.0036	& Kiss \textit{et al.} (1999) \\
2451110.3465 &   14029 &   $-$0.0066	& Kiss \textit{et al.} (1999) \\
2452093.5303 &   19313 &   +0.0025	& present paper \\
2452098.5517 &   19340 &   +0.0001	& present paper \\
2452101.5252 &   19356 &   $-$0.0035	& present paper \\
2452110.4585 &   19404 &   $-$0.0013	& present paper \\
\hline
\end{tabular}
\end{center}

\vspace{1.0cm}

\textit{Acknowledgements}: I wish to express my gratitude to J. Delgado Pin for writing a computer program to determine maximum timings for asymmetric light-curves, and to Joan A. Cano and Rafael Barber\'a for writing the software for obtaining and reducing the CCD frames. I wish also thank E. Rodr\'{\i}guez from Instituto de Astrof\'{\i}sica de Andaluc\'{\i}a for kindly providing me with information about the nature of V2109 Cyg, and E. Garc\'{\i}a-Melendo for his discussion of the results.

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\references

ESA, 1997, {\it The Hipparcos and Tycho Catalogues}, ESA SP-1200

Hauck, B., Mermilliod, M., 1998, {\it A\&AS}, {\bf 129}, 431

Kazarovets, A. V., Samus, N. N., Durlevich, O. V., Frolov, M. S., Antipin, S. V., Kireeva, N. N., Pastukhova, E. N., 1999, {\it IBVS}, 4659

Kiss, L. L., Cs\'ak, B., Thomson, J. R., Vink\'o J., 1999, {\it A\&A}, {\bf 345}, 149

Olsen, E. H., 1983, {\it A\&AS}, {\bf 54}, 55

Rodr\'\i guez, E., L\'opez-Gonz\'alez, M. J., L\'opez de Coca, P., 2000, {\it A\&AS}, {\bf 144}, 469

Rodr\'\i guez, E., 2002, Private communication

\endreferences


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