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\IBVShead{4364}{9 August 1996}

\title{NSV\,07968 IS AN OVERCONTACT ECLIPSING BINARY STAR}
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\begintext 

 The variability of NSV\,07968 (BV\,105, CSV\,007493) was announced by Geyer
et al. (1955) who indicated that this star was a possible Cepheid with a 
photographic magnitude variation from 10\fm 9 to 11\fm5. According to 
Kholopov (1982), the spectral type of this object is A6. NSV\,07968 can be 
unambiguously identified with GSC 3070.0345, a star with a photovisual 
magnitude (PAL-V1 filter) of 9.92$\pm$0.40.

 For 10 nights, from 6 May 1996 to 21 May 1996, NSV\,07968 was observed in the
V band using the 0.4-m telescope at Mollet del Valles Observatory (Spain) 
and a Starlight Xpress CCD camera. As comparison star was used GSC\,3070.0265
with a photovisual magnitude (PAL-V1 filter) of 10.11$\pm$0.40.

 Observations show that NSV\,07968 is not a Cepheid but an overcontact eclipsing
binary star with a period over 8 hours. Phase curve shows that primary
minimum is a transit with a 0\fm 28 depth and secondary minimum is
an occultation with an average depth of 0\fm 25. It was also observed
that Min.\,I has a distorted shape whereas shape and depth of Min.\,II
changed from cycle to cycle during the observational period.

 The following ephemeris was derived:
\vskip 2mm
\centerline{Min.\,I = HJD\,2450219.5238 + 0\fd 3825$\times$E}
            \hspace*{72mm} $\pm$0.0010 \ $\pm$0.0002
\vskip 2mm
 Light curve was preliminary solved using Binary Maker 2.0 (Bradstreet, 1993).
For this purpose, the phase curve was reduced to 100 normal points by 
dividing it into 100 non overlapping identical intervals, and averaging 
individual observations within each interval. Average points were finally
converted to flux. In computing the photometric solution the following
parameters were initially adopted: a mean surface temperature T$_1$=8300K
according to the spectral type of the system, and gravity darkening 
coefficients g$_1$=g$_2$=1 and bolometric albedos A$_1$=A$_2$=1, which correspond
to stars with radiative external layers. Limb darkening coefficients x$_1$ and
x$_2$ were set to 0.5.

 Since there is no spectroscopic information about mass ratio, a search for
the solution was carried out from q=0.13 to q=0.07, in mass ratio steps 
of 0.01 between q=0.13 and q=0.10, 0.005 between q=0.1 and q=0.08, 
and 0.0025 between q=0.08 and q=0.07. Simple inspection showed that there
were no adequate solutions out of the search interval. As best fit
criterion, minimum scatter of residual values obtained by subtracting the 
synthetic flux curve from normals points was used. Elements of the best 
solution are given in Table 1.  Figure 1 shows superimposed phase and 
synthetic curves. 

 Solution of the light curve of NSV 07968 indicates that this object
is an A-type W UMa system with one of the smallest known mass ratios. 
Additional spectroscopic and photometric observations should be carried out
in order to more accurately determine the physical parameters of this
binary star.
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\centerline{2}
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\centerline{Figure 1}

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\centerline{Table   1}

\medskip
\centerline{ mass ratio=0.0725$\pm$0.050}
\centerline{  i=90\degr$\pm$10\degr}
\centerline{ fill out=0.80$\pm$0.10}
\medskip
\hspace*{4cm} Omega$_1$=Omega$_2$=1.832$\pm$0.010

\hspace*{4cm} a$_g$=0.665$\pm$0.005\ \ \ a$_s$=0.261$\pm$0.010

\hspace*{4cm} b$_g$=0.646$\pm$0.006\ \ \ b$_s$=0.198$\pm$0.007

\hspace*{4cm} c$_g$=0.565$\pm$0.004\ \ \ c$_s$=0.188$\pm$0.007

\hspace*{4cm} d$_g$=0.742$\pm$0.013\ \ \ d$_s$=0.258$\pm$0.005

\hspace*{4cm} g$_1$=g$_2$= 1.0

\hspace*{4cm} x$_1$=x$_2$= 0.5

\hspace*{4cm} A$_1$=A$_2$= 1.0

\hspace*{4cm} T$_1$=8300K\ \ \ T$_2$=7950K$\pm$350K

\hspace*{4cm} L$_1$=0.9095$\pm$0.0010\ \ \ L$_2$=0.0905$\pm$0.008


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\beginauthors
Josep M. GOMEZ-FORRELLAD               
Grup d'Estudis Astronomics              
Apartado 9481                           
08080 Barcelona                       
Spain                                   
e-mail:jmgomez@astro.gea.cesca.es    
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\authorsrightcol                     
 Enrique GARCIA-MELENDO
 Esteve Duran Observatory
 El Montanya - Seva
08553 SEVA
 (Barcelona)
Spain
e-mail:duranobs@astro.gea.cesca.es
\endauthors
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\references

Bradstreet, D. H., 1993, Binary Maker 2.0 User Manual, Contact Software, 
 Norristown, Pennsylvania

Geyer, E., Kippenhahn, R., Strohmeier, W., 1955, {\it KVB}, No.\,11

Kholopov, P. N. editor, 1982, New Catalogue of Suspected Variable Stars,
 Moscow

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