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\input ibvs2.sty

\begin{document}

\IBVShead{5680}{25 January 2006}

\IBVStitle{The Fast Apsidal Motion System NSV 18773}

\IBVSauth{Otero, Sebasti\'an A.$^{1,2}$; Wils, Patrick$^3$}
\IBVSinst{Grupo Wezen 1 88, Buenos Aires, Argentina; e-mail: varsao@fullzero.com.ar}
\IBVSinst{Centro de Estudios Astron\'omicos (CEA), Mar del Plata, Argentina}
\IBVSinst{Vereniging Voor Sterrenkunde, Belgium; e-mail: patrick.wils@cronos.be}

\SIMBADobj{HD 99898}
\GCVSobj{NSV 18773}

\IBVStyp{ EA }
\IBVSkey{Variable stars, Eclipsing binary, Eccentric binary, Apsidal motion}
\IBVSabs{Combination of the publicly available ASAS-2 Ic and ASAS-3 V data shows}
\IBVSabs{that NSV 18773 is an eclipsing binary showing fast apsidal motion.}


\begintext

Recently the NSV catalogue (Kukarkin and Kholopov, 1982) and its supplement (NSVS) 
(Kazarovets et al., 1998) were searched for new eclipsing binaries (see Otero, 2003 for more details). 
After a candidate star was identified, its observations in the {\it ASAS-3} database (Pojmanski, 2002) 
were checked. This way, NSV~18773 = HD~99898; $V$ range 9.34 - 9.52 (Min~II= 9.50) was found to be 
an early-type (O9V according to Jaschek, 1978) EA-type system. It is also a visual binary, 
with components of magnitude 9.9 and 10.3$V_T$, and a separation of $0\farcs8$ (Fabricius et al., 2002). 
From the {\it ASAS} data alone there is no way to tell which one of the visual components is the eclipsing binary. 

A literature search revealed that the star was already listed in the {\it ASAS-2} catalogue 
(Pojmanski, 2000) as an eclipsing binary with a period of 5.048912 days. 
Combining the {\it ASAS-2} data ($I_c$ magnitudes were shifted to the $V$ values for the analysis) 
with the more recent {\it ASAS-3} observations (see Fig.~1) makes evident that this system shows very fast 
apsidal motion (Fig.~2) which makes it a specially interesting case. 
Fig.~3 reveals that no colour changes ($V-I_c$ = 0.49) occur during primary (Min~I) or secondary minimum (Min~II), 
which is consistent with the observed similar primary and secondary eclipses 
(0.17 and 0.15 mag. deep respectively) although the third light affects the results and the amplitude. 
The eclipses are partial, with a duration of Min~I= 0.566 days and of Min~II= 0.283 days (0.112 P. and 0.056 P. respectively). 
\\

The current light elements are:

	Min~I = HJD 2450563.528 + 5.04913 $\times$ E

	Min~II= HJD 2450566.202 + 5.04983 $\times$ E
\\

Table~1 shows times of minima and residuals for NSV~18773 as well as the phase of Min~II based on the primary eclipse 
period to make the phase shift of the secondary eclipse evident.

\begin{table}
\begin{center}
\caption{Recorded minima of NSV~18773.} \vskip2mm
\begin{tabular}{crlcc} 
\hline
HJD-2400000 & $O-C$    & Min & Phase & Source \\
\hline
50583.704 & $-$0.021 & I  &       & ASAS-2 \\
51136.811 &  0.057 & II & 0.541 & ASAS-2 \\
51194.673 &  0.004 & I  &       & ASAS-2 \\
51232.731 &  0.044 & II & 0.538 & ASAS-2 \\
51303.502 &  0.127 & II & 0.555 & ASAS-2 \\
51376.483 &  0.045 & I  &       & ASAS-2 \\
51525.741 &  0.204 & II & 0.570 & ASAS-2 \\
51926.775 & $-$0.018 & I  &       & ASAS-3 \\
52103.488 & $-$0.025 & I  &       & ASAS-3 \\
52116.482 &  0.197 & II & 0.569 & ASAS-3 \\
52411.500 & $-$0.010 & I  &       & ASAS-3 \\
52512.482 & $-$0.010 & I  &       & ASAS-3 \\
52646.756 &  0.312 & II & 0.591 & ASAS-3 \\
52651.793 &  0.300 & II & 0.589 & ASAS-3 \\
52739.699 & $-$0.004 & I  &       & ASAS-3 \\
52747.725 &  0.299 & II & 0.589 & ASAS-3 \\
53052.731 & $-$0.018 & I  &       & ASAS-3 \\
53365.844 &  0.049 & I  &       & ASAS-3 \\
53444.662 &  0.456 & II & 0.620 & ASAS-3 \\
53547.560 & $-$0.004 & I  &       & ASAS-3 \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{8cm}{5680-f1.eps}{Light curve of NSV~18773 showing {\it ASAS-2} and {\it ASAS-3} observations.}
\IBVSfigKey{5680-f1.eps}{NSV 18773}{light curve}

\IBVSfig{8cm}{5680-f2.eps}{Phase shift in the secondary eclipse of NSV~18773.}
\IBVSfigKey{5680-f2.eps}{NSV 18773}{light curve}

\IBVSfig{8cm}{5680-f3.eps}{$V-I_c$ light curve of NSV~18773 showing no colour changes during eclipses.}
\IBVSfigKey{5680-f3.eps}{NSV 18773}{light curve}

\vspace*{-0.7cm}

Assuming an inclination of nearly 90 degrees, 
the current ratio of eclipse durations of 0.50 
and the phase of Min~II with respect to Min~I (from Table~1)
can then be used in the formulae given by Kallrath and Milone (1999), allowing 
the eccentricity $e = 0.36 \pm 0.03$ and apsidal period $U = 135 \pm 10$ years to be estimated.  
The uncertainties for these values have been estimated by varying the eclipse duration ratio and
the phases of Min~II within their uncertainty limits.


{\bf Acknowledgements:} 
The authors thank John Greaves for his collaboration and suggestions. 
This research has made use of the SIMBAD and VizieR databases operated at the 
{\it Centre de Donn\'ees Astronomiques} (Strasbourg, France).

\references

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%%(2002A\&A...384..180F)

Jaschek, M., 1978, {\it Bull. Inform. CDS}, {\bf 15}, 121, Catalogue of selected spectral types in the MK system 
%%(1978BICDS..15..121J)

Kallrath, J., Milone, E.F., 1999, New York : Springer, Eclipsing binary stars : modeling and analysis 
\ADSBIBCODE{1999ebs..conf.....K}

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%%(1998IBVS.4655....1K)

Kukarkin, B.V., Kholopov, P.N., 1982, New Catalogue of Suspected Variable Stars,
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\ADSBIBCODE{1982ncsv.book.....K}

Otero, S., 2003, {\it IBVS}, 5480 

Pojmanski, G. 2000, {\it Acta Astronomica}, {\bf 50}, 177, The All Sky Automated Survey. Catalog of about 3800 Variable Stars 
%%(2000AcA....50..177P)

Pojmanski, G., 2002, {\it Acta Astronomica}, {\bf 52}, 397, The All Sky Automated Survey 
%%(2002AcA....52..397P)

\endreferences

\end{document}

