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\IBVShead{5106}{9 June 2001}

\IBVStitletl{Discovery of Pulsations in A5(8) V Component}{ of the Algol-type System TW D\lowercase{ra}}

\IBVSauth{Kusakin, A. V.$^1$; Mkrtichian, D. E.$^2$; Gamarova, A. Yu.$^2$}

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

\IBVSinst{Astronomical Observatory, Odessa National University,
Shevchenko Park, Odessa, 65014, Ukraine}

\SIMBADobj{TW Dra}
\IBVStyp{ DSCT }
\IBVSkey{stellar pulsations}

\begintext

According to the strategy of Central Asian Network (CAN) collaboration
(Mkrtichian et al., 1998) we are carrying out the survey for search
for and study of new pulsating components in eclipsing binary stars. In
the previous two publications (Mkrtichian and Gamarova, 2000; Gamarova
et al., 2000) we reported about our first discoveries of new pulsating
components in eclipsing binary systems R CMa and AS Eri. In this paper
we present our third detection of $\delta$ Scuti-type pulsation in the
primary component of the semi-detached eclipsing binary system
TW Dra.

TW Dra is a semi-detached binary system with A5(8) V primary and K0 III
secondary components. According to spectral class the primary component
of TW Dra is situated inside the instability strip and was included to
our list of target stars. Photoelectric observations of TW Dra through
Johnson $V$ filter using comparison HD 138852 ($V=5.758$, Sp = K0III)
and check HD 139549 ($V =9.13$, Sp = F8) stars were carried out on
April 26/27 and 28/29 2001 (JD 2452026, JD 2452028) with the 0.48-m
telescope at Tien-Shan Astronomical Observatory (Kazakhstan). The data
were reduced using standard reduction procedures for differential
data. The magnitude differences between TW Dra and HD 138852 folded
with the orbital period are shown on Fig.~1. The phases of orbital
period were calculated according to the GCVS ephemeris
HJD(Min~I)${}=2444136.2956+2.806847\times E$ (Kholopov et al., 1985).

Our observations covered the descending branch of the primary minima
(JD 2452026) and out-of-eclipse part of the orbital light curve (JD
2452028) (see Fig.~1). For search for and analyse short-periodic
pulsational variability we removed the orbital trends from the light
curve. The pulsational light curves of the two nights are plotted in
Fig.~2. The small-amplitude variations appear during both nights
including the night which corresponds to the descending branch of
primary minima.

The time series analyses were carried out with Kurtz's modification
(Kurtz, 1985) of the Discrete Fourier Transform (DFT) algorithm of
Deeming (1975). For determination of accuracy of the obtained
values of frequency and amplitude of pulsations we used routines
``Four'', which realizes least-squares multi-frequency method of
differential corrections fitting the multi-frequency signal
simultaneously with set of given frequencies (Andronov, 1994). The
amplitude spectrum of the two nights is shown in the top panel of
Fig.~3. The highest peak at $17.99\pm0.02$ c/d ($P=0\fday0556$) with
semi-amplitude of about $2.1\pm0.3$ mmag is well visible and confirms
the presence of pulsation. The amplitude spectrum of the residual is
shown in the bottom panel of Fig.~3. It does not show any prominent
peak above the noise level. The sine-wave fit with the period of
0\fday0556 for both nights is shown in Fig.~2 by a solid line. The
phase curve folded with the same period is shown in Fig.~4.

Adopting the $M = 1.7M_{\odot}$ and radius $R = 2.4_{\odot}$ for TW
Dra (Svechnikov and Kuznets\-ova, 1990) we determined the mean density of
the primary $\rho/\rho_\odot=0.123$ that gives the pulsation constant $Q
= 0.0190$. This value is close to the 2nd overtone of low $\ell$-degree
modes (Fitch, 1981).

\IBVSfig{10.5cm}{5106-f1.eps}{The orbital light curve}


\IBVSfig{4.5cm}{5106-f2.eps}{The pulsation light curves. Orbital trends are removed}

\IBVSfig{10cm}{5106-f3.eps}{The amplitude DFT spectra}


\IBVSfig{10cm}{5106-f4.eps}{The phase curve folded with the period of 0\fday0556}

\clearpage

\references

Andronov, I. L., 1994, {\it Odessa Astronomical Publications}, {\bf 7},
49, part 1
%Changed: Tue May 31 15:08:06 MEST 2005

Deeming, I. J., 1975, {\it Astrophys.\ \& Space Sci.}, {\bf 36}, 137

Fitch, W. S., 1981, {ApJ}, {\bf 249}, 218

Gamarova, A. Yu., Mkrtichian, D. E. and Kusakin, A. V., 2000, {\it IBVS}, No.\ 4837

Kholopov, N. P. et al., 1985, General Catalogue of Variable Stars, 4th Edition,
Moscow

Kurtz, D. W., 1985, {\it MNRAS}, {\bf 213}, 773

Mkrtichian, D. E., Kusakin, A. V., Janiashvili, E. B., Lominadze, J. G.,
Kuratov, K., Kornilov, V. G., Dorokhov, N. I. and Mukhamednazarov, S.,
1998, {\it Contrib.\ Astron.\ Obs.\ Skalnat\'e Pleso}, {\bf 27}, 238

Mkrtichian, D. E. and Gamarova, A. Yu., 2000, {\it IBVS}, No.\ 4836

Svechnikov M.~A. and Kuznetsova E.~F., 1990, The Catalogue of the
Approximate and Absolute Elements of Eclipsing Variable Stars, {\bf
1--2}, Sverdlovsk, Publications of Ural University

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