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\IBVShead{4314}{25 March 1996}

\centerline {\ft MULTIPERIODICITY IN PULSATING LAMBDA Boo STAR}
\centerline {\ft 29 Cyg (HD\,192640, V1644 Cyg)}

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\begintext

\bigskip

The first and firm detection of pulsation phenomena in $\lambda$ Boo
type stars
was made for classical  $\lambda$ Boo star 29 Cyg by Gies \& Percy (1977).
Gies \& Percy (1977) (G\&P) reported about the 45 min
variability of 29 Cyg with an amplitude of about 0.03 mag. They suggested
that, in contrary to Am and Ap stars lying in the $\delta$ Scuti instability
strip, the moderate amplitude pulsations cannot be an exclusion in
$\lambda$ Boo group.   Until the early 90-ies  few attempts
were carried out for investigation of
pulsational activity of this group of stars or the attempts were made
for nonclassified members of this group as for  suspected $\delta$ Sct-type
stars.  At present, up to 10 pulsating members are known (Kuschnig et al.
1994).

The new observations of 29 Cyg  were obtained during eight nights of July-September
1995 within the frames of collaborative program of the Odessa
Astronomical Observatory (Ukraine),  the  Sternberg State Astronomical
Institute (Russia) and Tien Shan High Altitude Observatory (Kazakhstan)
for the search and investigation of pulsating
northern chemically peculiar and $\delta$ Scuti type stars.
We used a 4-channel W,B,V,R photometer (Kornilov \& Krylov, 1990)
attached to the 0.48m telescope of the Tien-Shan High Altitude Observatory.
  The integration time was 10 seconds simultaneously in the four
filters and the data were binned in 40 second time intervals prior to the analysis.
The comparison stars HD\,192661 (G8 III, V=6.567) and HD\,192538 (A0V,
V=6\fm 468) were used.

This brief note represents the periodicity analysis of B$-$R index data
less affected by the influence of the marginal weather conditions,
the atmospheric scintillation noise and extinction variations.
This technique is based on the fact that the atmospheric scintillation
noise is strongly coherent in different wavelength bands, and amplitudes of
variability are maximal in the blue region of spectra of A stars and
decrease  at the long wavelengths.

Figure 1 shows the resulting instrumental B$-$R  data, for JD 2449920,  JD\,2449943,
JD\,2449949, JD\,2449955, JD\,2449956, JD\,2449957, JD\,2449963, and JD\,2449969 reduced to the mean level.
The analysis of separate nights shows that the main periodicity
at  the frequency near 37.5 c/d  is present in the B$-$R data.


The amplitude spectra of B$-$R data combined for 8 nights of photometry
are given in Figure 2. The highest peak with semi-amplitude of about 6 mmag
is present at frequency 37.43 c/d (38.47 min).  After the application of
consecutive prewhitening procedure and frequency analysis we have
resolved  up to 7 frequencies. The amplitudes of frequencies are
slightly variable from night to night. The frequencies f6 and f7 can be the
artifacts produced by the variability of main frequencies 37.43 and 29.43 c/d. Frequencies, amplitudes
and phases were corrected by the application of the simultaneous
7-frequency iterative sine-wave least-squares fitting procedure.

The resulting frequencies and amplitudes are given in Table 1.
The 7-frequency synthetic curve is shown in Figure 1 by the solid line.

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\centerline{2}
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\centerline{Table 1. Frequency fit for JD\,2449943 - 2449969 (B$-$R) and G\&P V data sets}
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\begin{center}
\begin{tabular}{|ccc||ccc|}
\hline
 & (B$-$R) data &  (Present paper)& & V data &(Gies \& Percy, 1977)\\
  &Freq. (c/d)& Semi-ampl.(mag) & &Freq.(c/d)&Semi-ampl.(mag)\\
\hline
f1 & 37.43&0.0051&f1&29.59 &0.0085\\
f2 & 29.43&0.0031 &f2&37.86&0.0069\\
f3 & 5.73&0.0024 &f3& 20.09 &0.0041\\
f4 & 26.19&0.0032 &f4&12.02 &0.0036 \\
f5 & 12.85&0.0025 &f5& 28.86&0.0030\\
f6&31.71 & 0.0024&& & \\
f7&38.03&0.0018&&&\\
\hline
\end{tabular}
\end{center}
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\centerline{\epsffile{4314-f1.ps}}
\begin{center}
Figure 1. The B$-$R light curves for 29 Cyg. The solid line is the 7
frequency fit (see Table 1)
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\centerline{3}
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\centerline{\epsffile{4314-f2.ps}}
\centerline{Figure 2. The amplitude spectra of B$-$R data shown in Figure 1}
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In order to confirm the frequencies detected we re-analysed the Gies
and Percy's 1976 year data set (Gies and Percy, 1977).
Our analysis of the G\&P V filter data after removing the trends
produced by the 1.54 c/d variability of comparison Be star
28 Cyg (Pavlovski et al., 1993) gives the 5 frequency solution
also presented in Table 1.
 The comparison of two solutions shows that at least 3 frequencies
in our data set are close to those presented in G\&P data.  In contrary
to our 1995 year observation,  the maximal amplitude of
variability of G\&P data set is at frequency of 29.59 c/d (48.7
min). For G\&P data set the amplitude ratio for the mean amplitudes
of two  frequencies 37.5  and 29.5 c/d is equal to 0.81.
 For our data set this ratio is equal to 1.65, i.e.  this result is
a  confirmation  of amplitude variability of excited modes
in 29 Cyg, and this fact of amplitude variability is not an exclusion for stars which occupy the  blue edge of
instability strip (see the extreme case of mode variability in
HD\,74292 (A2V) in paper of Kusakin \& Goranskij, 1995).

The  main results of our brief note are as follow:
\begin{itemize}
\item 29 Cyg is a pulsating $\lambda$ Boo star with a multiperiodic structure of
excited modes, the two highest amplitude excited modes have the frequencies
near  37.5 and 29.5 cycles per day.
\item We confirm at least 3 of detected frequencies by the
re-analysis of 3 nights of Gies \& Percy (1977) V data for 29 Cyg.
\item The amplitudes of the excited modes are variable.
\end {itemize}

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\beginauthors
\authorsrightcol
A.V. KUSAKIN
Sternberg State Astronomical
Institute, Universitetsky prospect, 13
Moscow, 119899, Russia
\vskip 1mm
D.E. MKRTICHIAN
Astronomical Observatory,
Odessa State University,
Shevchenko Park, Odessa,
270014, Ukraine.
e-mail: david@oao.odessa.ua
\endauthors

\references

Gies, D.R. and Percy, J.R., 1977, {\it AJ}, {\bf 82}, 166

Kornilov, V.G. and Krylov, A.V., 1990, {\it Russian Astron. J.}, {\bf
67}, 173.

Kusakin, A.V. and Goranskij, V.P., 1995, {\it Russian Astron. J.},
(in press)

Kuschnig, R., Paunzen, E. and Weiss, W.W., 1994, {\it IBVS}, No.\,4070

Pavlovski, K., Ruzic, Z. and Vujnovic, V., 1993, {\it ASP Conference
Series}, {\bf 40}, 724, {\it Inside the Stars},
eds. Weiss, W.W. and Baglin, A., IAU Colloquium 137
%Corrected: Wed Jun  1 16:03:28 MEST 2005


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