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\IBVShead{5087}{23 May 2001}

\IBVStitle{Stability of Pulsation of V577 Ophiuchi}

\IBVSauth{Zhou, Ai-Ying}

\IBVSinsto{National Astronomical Observatories, Chinese Academy of Sciences,
Beijing, P.R. China\\ \indent e-mail: aiying@bao.ac.cn}

\SIMBADobj{V577 Oph}
\IBVStyp{ DSCT }
\IBVSkey{CCD photometry}
\IBVSabs{This note presents the light curves for V577 Oph}
\IBVSabs{collected at BAO in 2000}

\begintext

V577 Ophiuchi (11.01~$V$, A8) is an eclipsing binary with a
non-circular orbit (eccentricity $e =0.22 \pm 0.08$) and a $\delta$
Sct-type primary component (Diethelm 1993).  Previous observations
clearly show the intrinsic pulsational variation superimposed on the
eclipse light curve. The pulsation just disappears in the primary
minimum and has almost double amplitude in the secondary one (Volkov
1990).  In order to detect the multiperiodicity of the pulsation of
V577 Oph, we observed the star in 2000.  The data were collected with a
red-sensitive Thomson TH7882 $576 \times 384$ CCD photometer (Wei et
al.\ 1990; Zhou et al.\ 2001) on the 85-cm Cassegrain telescope at the
Xinglong Station of the Beijing Astronomical Observatory of China.  The
CCD has an imaging size of $13.25\times8.83$~mm$^{2}$ corresponding to
a sky field of $12\farcm3 \times 8\farcm4$ (1\farcs2/pixel, a pixel
size is 23~$\mu$m$^{2}$).  Two stars in the field of V577 Oph were
selected as references. They are

C1 = GSC 00444\_01191 ($RA = 18\hr 16\mm 46\fsec 55$,
$DEC = 06\deg 57\arcm 08\farcs2$, 2000.0, 12.6~$V$),

C2 = GSC 00444\_02025 ($RA = 18\hr 17\mm 02\fsec51$,
$DEC = 06\deg 54\arcm 37\farcs6$, 2000.0, 11.5~$V$).

Exposure times were 30 s. A Johnson $V$ filter was used.  Atmospheric
extinction was not taken into account in view of the close spacing of
the observed stars. The differential colour effect between the variable
and the reference stars are largely eliminated by taking the mean
combination of the latter.  Hence the differential magnitudes of V577
Oph are calculated relative to the two comparison stars as ${\rm
V-(C1+C2)}/2$.  The magnitude differences between the comparison stars
generally show a typical standard deviation of 0\fmm010.  For the
nights of good seeing a better value of about 0\fmm006 was obtained.
These two comparison stars were detected to be non-variables at the
accuracy of observation.

A preliminary Fourier analysis based on the data from four nights (11,
12, 21 and 30 June 2000) demonstrates that the light variations of V577
Oph can be well fitted with a single pulsation frequency $f =14.3903$
cycle\,d$^{-1}$ ($P=0.069491$ d) having a semi-amplitude of 0\fmm0289.
The fitting yields the residuals with a standard deviation of $\sigma
=0\fmm0162$.  We tried fitting the light curves with two more
frequencies.  When an additional second term at 16.2738 cycle\,d$^{-1}$
was considered, the $\sigma$ or the quality of fitting was not
significantly improved.  We recalled that all the observations
collected in 1987--1990 by Volkov (1990) revealed the stability of
pulsation in 2.5 year interval period. The period given in the
ephemeris HJD$_{\rm max} = 2447620.379 + 0.0694909\times E$ (Volkov
1990) is in close agreement with the present value.  Therefore, we
think that the pulsation frequency of V577 Oph has been quite stable since
1987. The question of amplitude variability is still open.  Figure~1
displays the observed (dots) and fitted (lines) light curves of the
star from the four nights.

\newpage

\IBVSfig{16cm}{5087-f1.eps}{CCD differential light curves of V577 Oph
from four nights in June 2000}

{\it Acknowledgements.} This work was supported by the Natural Science
Foundation of China.

\references

Diethelm, R., 1993, {\it IBVS}, No.\ 3894

Volkov, I. M., 1990, {\it IBVS}, No.\ 3493

Wei, M.-Z., Chen, J.-S., \& Jiang, Z.-J., 1990, {\it PASP}, {\bf 102}, 698

Zhou, A.-Y., Rodr\'{\i}guez, E., Liu, Z.-L., \& Du, B.-T.,
2001, {\it MNRAS}, in press

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