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\IBVShead{4726}{16 June 1999}

\IBVStitle{NSV 13826: a {\Large $\beta$} Lyrae Type Eclipsing Binary}


\IBVSauth{J. VANDENBROERE}

\IBVSinsto{Groupe Europ\'een d'Observations Stellaires (GEOS), 3 Promenade
Venezia, F-78000 Versailles.\\ E-mail: j.vandenbroere@skynet.be }

\SIMBADobj{NSV 13826}
\IBVStyp{ EB }
\IBVSkey{photometry}
\IBVSabs{NSV 13826 was found to be an eclipsing binary on the basis of}
\IBVSabs{visual estimates and photoelectric measurements.}
\IBVSabs{The $V$ and $B-V$ light curves also show some peculiarities.}

\begintext

NSV 13826 ($\equiv$ GSC 2189.704 $\equiv$ HV 06152 $\equiv$ P 5616) is catalogued as a possible eclipsing or irregular variable from a list published by Shapley and Hughes (1934) and from irregular variations detected by Sandig (1950) on photographic plates. The $\beta$ Lyrae shape of its light curve was obvious from the visual estimates of several GEOS members and its orbital period appeared to be
1.0803 days (Vandenbroere, 1996). 

\IBVSfig{9.25cm}{4726-f1.ps}{$V$ and $B-V$ (Geneva system) light curves of NSV
13826}

\vskip-11pt
\IBVSfig{9.25cm}{4726-f2.ps}{Different behaviours were observed in the 0.65--0.73 phase interval}

This value was used to collect new and more accurate photoelectric data.
Therefore, NSV 13826 (2\hr39\mm28\sec, +23$\deg$01\farcm5, 2000.0) was measured with the 76-cm telescope of the Jungfraujoch station through 
the $B$ and $V$ filters of the Geneva system. 144 measurements were 
obtained in each filter between January 1995 and December 1998. The folded $V$ and $B-V$ light curves are shown in Fig.~1. NSV 13826 is a $\beta$ Lyrae star with $V=10\fmm12$ at maximum light, a primary minimum as faint as $V=10\fmm59$ and a secondary one going down to $V=10\fmm35$. The $(B-V)_G$ colour indices vary slightly from $-0.59$ to $-0.50$, which corresponds to a $(B-V)_J$ index of 0.31 to 0.38 after transformation  with the formulae described by Meylan and Hauck (1981) and using the luminosity class~III.

A more accurate orbital period was calculated by means of the primary and secondary photoelectric minima and of 30 instants extracted from the visual estimates of three GEOS members. The result of the linear regression, giving a triple weight to the photoelectric moments, is:
$$ \begin{array}{r@{}r@{}l}

{\rm Min~I = HJD~} 2449639.835 & {} +  1\fday080313 & {} \times E. \\
                      \pm0.012 &        \pm0.000033 \\
\end{array}
$$

A close inspection of Fig.~1 shows irregularities in the shape of the light curve, particularly on the shoulders before both maxima. Note also the related behaviour of the $B-V$ curve which mimics the $V$ one. Probably the star is undergoing mass exchanges and short-time scale variability is responsible for the scatter around the 0.65-0.73 phase interval, as better evidenced in Fig.~2. 

The author wishes to thank N.~Beltraminelli, R.~Boninsegna, G.~Dine-Zepter and J.~Remis for helping in obtaining the photoelectric data, D.~Dalmazio and A.~Manna for their visual estimates.

\references 

Meylan G. and Hauck B., 1981, A\&AS, 46, 281

Sandig H.U., 1950, Astron.\ Nachr., 278, 181

Shapley H. and Hughes E.M., 1934, Harvard Annals, 90, 4, 163

Vandenbroere J., 1996, Note Circulaire GEOS NC 805

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