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\centerline{\ft OBSERVATIONS OF THREE $\lambda$ BOOTIS STARS}
\centerline{\ft BY USING DUAL CHANNEL PHOTOMETER}
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A small group of $\lambda$ Boo stars attracts astrophysicists' attention
owing to their controversial evolutionary status. These stars related
apparently to a short-lived evolutionary phase
are available for the development and control of the modern stellar
atmosphere theories (Gray \& Corbally, 1993).
The discovery of the $\lambda$ Boo stars' pulsations (Weiss  et al., 1994)
gives the opportunity to apply the
tools of asteroseismology for their profound investigation.
Observations of the variability of $\lambda$
Boo stars with different equipments are widely presented in IBVS
by the Vienna working group {\it Asteroseismology - AMS}.
It should be noted, that a high quality of  sky seeing and
instrumentation, and long time data series are needed for such observations
because the stars with relatively long periods, from 0.5 up to 4 hours,
have very low amplitudes, from 0.004 mag to 0.07 mag.

During 1994-1995 we have observed 3 stars  from the list of Gray
\& Corbally (1993) by using the dual-channel photometer (Dorokhov \&
Dorokhova, 1994) attached to the 0.8m  Ritchey--Chretien
telescope, situated in Central Asia, at the Mt. Dushak-Erekdag station of
Odessa Astronomical Observatory.

{\bf HD\,204041} was observed in Str\"omgren v-filter on two nights, 11 and 12 Oct., 1994,
and  {\bf HD\,38545} (C1 HD\,39098, C2 HD\,39019) was observed in Johnson B-filter on 20 and 22 Nov., 1995 in single channel by using 3-star mode (see Breger, 1992).
Our observations confirm nonvariability of both stars within the upper limits
which are higher by 0.001 mag than presented in the papers by Paunzen et al.
(1996) and by Kuschnig et al. (1996).

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Figure 1.
On the top panel: light curves for HD\,221756 and comparison star
HD\,221903 in Johnson's B, on the bottom panel:
the VAR -- COM curves for two nights 12 and 16 Nov 1995. All the curves
are presented as residuals to the corresponding night--means.
The solid line is a least squares fit of $f_1$ and $f_2$ frequencies
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Figure 2. Fourier spectrum of both nights data. The middle panel shows
the same after removal for $f_1$=15.85 c/d, the lower panel --
the result of the prewhitening for $f_1$=15.85 c/d and $f_2$=27.54 c/d
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{\bf HD\,221756} was tested by Paunzen \& Handler (1996)
in August 1995. They obtained a period of 63 min and amplitude
6.6 mmag in Str\"omgren b. We observed HD\,221756 and a comparison star
HD\,221903 (m=8.3 mag, A0) simultaneously in dual channel mode of the photometer
on the nights 12/13 and 16/17 Nov 1995.The data were
acquired as continuous 10 sec integrations in Johnson's B filter, interrupted by the channel reductions about one time per hour.

Then the counts of the comparison star in channel 2 were reduced to the
sensitivity level of channel 1,
the data were corrected for coincidence counting losses,
the sky background contribution and the atmospheric extinction, and were
binned to 2 min integrations by taking 12-point averages.
Figure 1 shows the light curves of HD\,221756 and the comparison star,
and the differential data as  residuals to the nightly means for each date.
In order to decrease the differential data noise level, comparison
star's observations were smoothed by a rectangle filter with window size=3.
 The solid line in Figure 1 is a least squares
fit of two frequencies, which were revealed from subsequently prewhitened amplitude
Fourier spectrum of common series of data (Figure 2).  The packaged program PERIOD (Breger, 1990) was used for Fourier analysis. Two peaks at frequencies
$f_1$=15.85 c/d (P=1.51 hour, A=0.011 mag) and $f_2$=27.54 c/d (P=52 min, A=0.006 mag) could be influenced by a 1 c/d aliasing. The result needs in further control
because the variations of sky transparency may affect the such low-amplitude
light curves even in the case of dual--channel photometry.
Here we can only suppose that HD\,221756, as well as already known HD\,210111
(Paunzen et al., 1994) and 29 Cyg (Kusakin \& Mkrtichian, 1996) is another
example of the multiperiodicity of $\lambda$ Bootis stars like that taking place in $\delta$ Scuti stars.
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\beginauthors
\authorsrightcol
T.N. DOROKHOVA
N.I. DOROKHOV
Astronomical Observatory 
Odessa State University
Shevchenko Park, Odessa 270014 
Ukraine
\endauthors
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\centerline{3}
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\references

Breger, M., 1990, {\it Commun. Asteroseismology}, {\bf 20}, 1

Breger, M., 1992, {\it Commun. Asteroseismology}, {\bf 46}, 1

Dorokhov, N.I., Dorokhova, T.N., 1994, {\it Odessa Astronomical Publ.}, {\bf 7}, 168

Gray, R.O., Corbally, C.J., 1993,  {\it AJ}, {\bf 106}, 632

Kusakin, A.V., Mkrtichian, D.E., 1996, {\it IBVS}, No.\,4314

Kuschnig, R., Gelbmann, M, Paunzen, E., Weiss, W.W., 1996, {\it IBVS}, No.\,4310

Paunzen, E., Handler, G., Weiss, W.W., North, P., 1994, {\it IBVS}, No.\,4094

Paunzen, E., Handler, G., 1996, {\it IBVS}, No.\,4301

Paunzen, E., Weiss, W.W., Kuschnig, R., 1996, {\it IBVS}, No.\,4302

Weiss, W.W., Paunzen, E., Kuschnig, R., Schneider, H., 1994, {\it A\&A}, {\bf 281}, 797

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