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\IBVShead{4182}{13 April 1995}                      % Use it with dummy or empty arguments

% Call for macro \title {Title of the paper}
\centerline{\ft THE ORBITAL PERIOD OF THE ROSAT CATACLYSMIC}
\centerline{\ft VARIABLE S\,10932 COMAE BERENICES}
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S\,10932 Com, the optical counterpart of the ROSAT X-ray source RX J1239.5
+ 2108, was -- by observations on Sonneberg photographic plates -- not only
found to have high (17\fmm) and low (18\fmm) brightness states like magnetic
cataclysmics; there could be noticed also an eruption (13\fm 5) similar to
that of a dwarf nova (Richter and Greiner, 1995). The only star hitherto known 
to show a similar behaviour is V426 Oph (Wenzel and Splittgerber, 1990). May be
that S\,10932 is related to this object, whose exact physical nature is, 
however, still poorly known.

Additionally, large fluctuations in the above mentioned states of high minimum
light and apparently also in the rare low states were observed. We therefore 
decided to carry out time-resolved differential photometry of S\,10932 at the
Sonneberg 60/180 cm reflector by means of a CCD camera of Wright Instruments
Ltd, type EEV 02-06-1-206. Three night series of ``repeated exposures'' in the
R band led to the detection of an eclipsing variability. The elements of
brightness variations are:
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\centerline{Min.(hel.) = J.D.\,244\,9486.4821 + 0\fd 08703 $\times$ E}
\hspace*{91mm}$\pm$\ 0.00002
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 Figure 1 shows the greater part of our series of 1994 May 15/16; $\Delta$m
 means the magnitude difference against one of the our comparison stars.
 The R band amplitude of the eclipses is about 1.5 mag and their duration 
 approximately 10 min. In the following table the five minima observed in the
 three nights of 1994 and the deviations from the ephemeris are listed:
 \vskip 1mm
 \begin{center}
 \begin{tabular}{crr}
 \hline
 HJD\,2440000+&E&O$-$C\\
 \hline
 \\
 9484.3934&$-$24&\ 0\fd 0000\\
 9484.4807&$-$23&+0.0003\\
 9486.4816&0&$-$0.0005\\
 9488.3969&22&+0.0001\\
 9488.4841&23&+0.0002\\
 \hline
 \end{tabular}
 \end{center}
\noindent Figure 2 gives all eclipse observations, folded by the above elements.
 
 Interestingly, the period length is just at the lower boundary of the well-known
 period gap of cataclysmic variables (0\fd 088...0\fd 118).
 
 Most objects with period lengths similar to that of S\,10932 are either SU
 Ursae Maioris or AM Herculis stars. S\,10932, however, is an outstanding 
 object and deserves further attention.
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 \centerline{2}
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\centerline{\epsffile{4182-f1.ps}}
 \centerline{Figure 1. Geocentric observing time (s after beginning of the
 current Julian Day)}

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\centerline{\epsffile{4182-f2.ps}}
 \centerline{Figure 2}

 Acknowledgements. We are most grateful to J. Greiner (Max-Planck-Institut
 f\"ur Extraterrestrische Physik, Garching) for stimulating the search for
 optical variability of ROSAT sources. Our work was partly supported by funds
 of German Bundesministerium f\"ur Forschung und Technologie under contract
 nos. 05-2A052A (W.W.) and 05-2S0524 (G.A.R.).
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% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
 W. WENZEL
 G. A. RICHTER
 R. LUTHARDT
 Sternwarte
 D-96515 Sonneberg
 Germany
\authorsrightcol
 R. SCHWARTZ
 Gleimstrasse 21
 D-10437 Berlin
 Germany
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\endauthors

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\references

 Richter, G. A., and Greiner, J., 1995, Proc. of the Padova-Abano Conference
 on Cataclysmic Variables, in the press
 
 Wenzel, W., and Splittgerber, E., 1990, {\it Inf. Bull. Var. Stars}, No.\,3532
 
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