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\IBVShead{4452}{4 March 1997}

\title{CCD PHOTOMETRY OF ECLIPSING BINARY AL~Ophiuchi}

\begintext

The eclipsing binary AL~Ophiuchi (= GSC\,0999.155;
$\alpha$(2000) = 17\fh 26\fmm 45\fss 4,
$\delta$(2000) = +12\degr 57\farcmm 56\farcss)
is a neglected, rather faint binary near $\alpha$~Oph.
Its $V$ magnitude given in the {\it Guide Star Catalogue} is 13\fm 05.
Unfortunately, this variable has not been studied photometrically for more
than 20~years and the light elements of this system are not given in
the~GCVS.

Variability of this star with amplitude 14.1 -- 15.0~mag was
discovered photographically by Reinmuth~(1926) in Heidelberg.
The spectral type of AL~Oph was measured by
Bond and Tifft~(1974) during their spectroscopic survey of some
high-latitude blue variables. They found a spectrum of~G5.
Meinunger~(1981) concluded that AL~Oph belongs to the W~UMa type
and the amplitude is too small for the period determination from
the older photographic measurements. Recently, the variability of
AL~Oph was examined on the plates of the Odessa Observatory by
V.I.~Marsakova (Andronov~1996) and several weakenings were obtained.
This star was also measured by Paschke~(1996). All previous measurements
lead to uncertain conclusions about its type and light elements.

The present CCD photometry of AL~Oph was carried out during 14~nights
in the period from June to November 1996 at the Ond\v{r}ejov Observatory,
Czech Republic, using a 65cm reflecting telescope with a CCD-camera
(SBIG~ST-6) in the primary focus.
The measurements were done using the standard Cousins $R$ filter
with exposure time from 45~to 120~s. Two nearby stars
GSC~0999.1235 ($V$ = 11.6 mag) and GSC\,0999.388  ($V$~=~12.8~mag)
on the same frame as AL~Oph served as a comparison and check stars
(Figure~1). Some of the observations were done through thin clouds.
The CCD data were reduced using software developed by
P.~Pravec and M.~Velen (Pravec et al.~1994). No correction
of relative magnitudes was allowed for airmass due to the
proximity of the comparison star to the variable (46~arcsec).
Deviations caused by differential extinction in the broad-band
filter for different colours of stars should not be significant.
Due to preliminary period close to one sidereal day, the primary
minima were observable only before the end of June~1996.
On August~25 we obtained a flat secondary minimum and,
fortunately, in November~21 L\v{S} observed a part of the
descending branch to the primary minimum.

The times of minimum and period were determined using
a new method of iterative least squares polynomial fitting.
The method for a minimum determination was developed
especially for processing of precise lightcurves with
partial coverage of both branches. This is a typical result
of non-automatical CCD observations, when several objects
are followed and for a particular object we obtain
groups of consequent images separated by large time
intervals. This method should provide also more reliable
results in the case when the number of points on each
branch differs. We suppose only the symmetry of the minimum.
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\centerline{2}
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\centerline{Figure 1. Finding chart of AL~Oph. The comparison and check
stars are also plotted}
\large

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\centerline{Table 1. New times of minimum of AL~Oph}
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\begin{center}
\begin{tabular}{llcccc}
\hline
  JD Hel.$-$   & Error   & Min. &  $O-C$  & Epoch &  $N$ \\
  24 00000   & [days]  & type &  [days] &       &      \\  \hline
  50250.3865 & 0.0004  & Pri. & +0.0006 &  7.0  &  16  \\
  50252.3717 & 0.0003  & Pri. &--0.0002 &  9.0  &  25  \\
  50321.3896 & 0.0009  & Sec. & +0.0038 & 78.5  &  65  \\  \hline
\end{tabular}
\end{center}

\vskip 3mm
Similar way of solution -- double iteration connected
with the least squares fitting -- can be used for
determination of light elements from several night
observation. We choose a symmetrical feature on the
lightcurve (primary minimum), estimate period and
basic light minimum. Then we fit a low-order polynomial
like in the case of simple minimum determination.
Varying the period we find a minimum residual corresponding
to the best period. Using this method we can determine the
precise value of period in the case of AL~Oph, where we
have only two primary minima with low accuracy in a short
time interval (see Table~1). We derived the following linear
light elements for the current use:

\begin{center}
Pri.Min. = HJD 24 50243.4348 + 0\fd 993005 $\times$ E. \\
$~~~~~~~~~~~~~~~~~~~~~~~\pm 0.0003\pm 0.000025$
\end{center}


\noindent Observed times of minima are presented in Table 1.
In this table, $N$ stands for the number of observations used
in the calculation of the minimum time, the other symbols are
self-explanatory. Figure 2 shows the composite differential $R$
light curve during the summer 1996. The light amplitude in $R$
colour for primary minimum according to our measurement is
$A_1 = 0.56 \pm 0.02$~mag, for secondary minimum we found
$A_2 = 0.10 \pm 0.02$~mag. The duration of both minima seems
to be about 2 hours. New measurements of this system are necessary
 to improve the above given elements.
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\centerline{Figure 2. Composite differential $R$-light curve of AL~Oph
obtained in 1996}
\large

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{\it Acknowledgment.} This work has been supported in part
by the Grant Agency of the Czech Republic, grant No. 205-95-1498 and
by the ESO C\&EE Programme, grant No. A-02-069. This research has
made use of the SIMBAD database, operated at CDS, Strasbourg, France.

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\beginauthors
Lenka \v{S}AROUNOV\'A
Astronomical Institute
Academy of Sciences
CZ-251 65 Ond\v{r}ejov
Czech Republic
Internet: {\tt lenka@asu.cas.cz}
\authorsrightcol
Marek WOLF
Astronomical Institute
Charles University Prague
CZ-150 00 Praha 5, \v{S}v\'edsk\'a 8
Czech Republic
Internet: {\tt wolf@mbox.cesnet.cz}
\endauthors

\references

Andronov, I., 1996, private communication

Bond, H.E., Tifft, W.G., 1974, {\it PASP}, {\bf 86}, 981

Meinunger, L., 1981, {\it Mitteilungen Ver. Sterne}, {\bf 9}, 59

Paschke, A., 1996, private communication

Pravec, P., Hudec, R., Sold\'an, J., Sommer, M., Schenkl, K.H.,
      1994, {\it Experimental Astronomy}, {\bf 5}, 375

Reinmuth, K., 1926, {\it Astronomische Nachrichten}, {\bf 226}, 193

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