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\IBVShead{5936}{27 April 2010} 

\IBVStitletl{Discovery and Photometric Orbital Solution of a New}{ Double-Lined and Highly Eccentric B5V Eclipsing Binary}

\IBVSauth{Siviero, A.$^1$; Dallaporta, S.$^2$; Munari, U.$^1$}

\IBVSinst{INAF Osservatorio Astronomico di Padova, Sede di Asiago, I-36032 Asiago (VI), Italy} 
\IBVSinst{ANS Collaboration, c/o Osservatorio Astronomico, I-36032 Asiago (VI), Italy}
 
\SIMBADobjAlias{GSC 3671-0099}{3UC 289-030898}
\IBVStyp{ Eclipsing binary } 
\IBVSkey{photometry, spectroscopy} 

\IBVSabs{We obtained a photometric orbital solution of a new, eccentric and}
\IBVSabs{double-lined B5V eclipsing binary, that we discovered in the field}
\IBVSabs{of the symbiotic system AX Per. During 131 nights extending from}
\IBVSabs{September 2006 to September 2009, we collected 155 photometric}
\IBVSabs{observations in B and 164 in V band. The orbital period is}
\IBVSabs{4.30885 days, the eccentricity 0.284, and the two nearly identical}
\IBVSabs{components are well within their Roche lobe to avoid to be tidally}
\IBVSabs{distorted. Radial velocities at three distinct epochs confirm the}
\IBVSabs{photometric orbital solution and fix to $-$101 km/s the barycentric} 
\IBVSabs{radial velocity.}


\begintext 

\vspace*{-0.5cm}

ANS (Asiago Novae and Symbiotic stars) Collaboration is monitoring in
$UBVR_{\rm C}I_{\rm C}$ bands the photometric evolution of the 80
symbiotic stars for which Henden and Munari (2000, 2001, 2006) provided
calibration photometric sequences.  While observing the symbiotic star AX~Per,
 it was noted that the star $b$ of the photometric sequence by Henden
and Munari (2006), located at RA=01\hr36\mm42\fsec4 and
DEC=+54\deg15\arcm21\farcs0 (J2000.0) and coincident with
 GSC~3671.0099 (= 3UC~289-030898 in the UCAC3 catalog),  
was indeed variable.  Henden
and Munari sequences, while accurately placed on the Landolt~(1992)
equatorial photometric standards, have been built from only three separate
epoch observations.  Unknown eclipsing binaries could easily pass undetected
when only few observations are available, and Henden and Munari warned
against the risk.

\IBVSfig{8.5cm}{5936-f1.eps}{The photometric observations from Table~1 are
plotted together with the light-curves corresponding to the photometric
orbital solution from Table~2.}
\IBVSfigKey{5936-f1.eps}{GSC 3671.0099}{light curve}

\begin{table}[!ht]
\centerline{{\bf Table 2.} Photometric orbital solution for  the eccentric eclipsing binary GSC 3671.0099.}
\vskip 3mm
\begin{center}
\begin{tabular}{lrclclrcl}
\hline
$T_0$(HJD)  &   2454158.301 & $\pm$ &   0.275 && $T_{\rm 2}(K)$  & 14480  & $\pm$ & 60    \\
$P$(d)      &       4.30885 & $\pm$ & 0.00001 && $\Omega_{\rm 1}$& 9.07   & $\pm$ & 0.08\\
Phase shift &         0.911 & $\pm$ &   0.064 && $\Omega_{\rm 2}$& 8.34   & $\pm$ & 0.10\\
$e$         &       0.284 & $\pm$ & 0.002 && $R_{\rm 1}$(R$_{\odot}$)&3.08&$\pm$& 0.03\\
${\omega}$(deg)&    201.67 & $\pm$ &   0.95 && $R_{\rm 2}$(R$_{\odot}$)&3.41&$\pm$& 0.05\\
\hline
\end{tabular}
\end{center}
\end{table}

After the serendipitous discovery of its variability (to which contributed
the ANS~Collaboration members G. Cherini, G.L. Righetti, S. Tomaselli, S.
Moretti, A. Vagnozzi and S. Bacci), we started an intensive monitoring of
GSC~3671.0099.  During 135 different nights, we obtained 201 observations in
$B$ and 212 in $V$ band with a 30cm telescope located in Cembra (Trento,
Italy) and equipped with a SBIG ST-9 CCD camera, 512$\times$512~array, 20
$\mu$m pixels $\equiv$ 1\farcs72/pix, with a field of view of
13\arcm$\times$13\arcm.  The $B$ filter was from Omega and the $V$
filter from Custom Scientific.  The data are given in Table~1 (available in
electronic form through the IBVS website as {\tt 5936-t1.txt}).  
\IBVSedata{5936-t1.txt}
\IBVSfigKey{5936-t1.txt}{GSC 3671.0099}{photometry}
A Deeming-Fourier analysis promptly revealed the
variability to be periodic with a period $P$=4.30885 days.  Figure~1 plots
the data in phase with the ephemeris for the primary minima $t_{min I} =
2454158.30072 + 4.30885\times {\rm E}$.  The light-curve shows GSC 3671.0099 to
be a highly eccentric eclipsing binary, with secondary eclipse occurring
photometric phase~0.33.  The mean brightness values away from eclipses are
$V=13.046$ and $B-V=+0.047$, and the depth of the primary eclipse is
$\Delta m=0.65$.  The star does not change color during both eclipses, or
away from them.

To classify the variable, a low resolution, wide wavelength range spectrum
of \\GSC~3671.0099 was obtained with the AFOSC imager+spectrograph mounted on
the Asiago 1.82m telescope.  The spectrum is presented in Figure~2. 
Comparison with the MK spectral atlas by Yamashita et al.  (1977) shows the
spectrum to be that of a normal B5V star.  The intrinsic color of a B5V star
is $(B-V)_{\circ}=-0.16$ (Fitzgerald, 1970), and therefore the
reddening affecting GSC 3671.0099 is $E_{B-V}=+0.21$.  This is close to
$E_{B-V}=+0.27$ generally accepted for the nearby symbiotic star AX~Per (Skopal
et al., 2001; Miko{\l}ajewska \& Kenyon, 1992).

\IBVSfig{5.6cm}{5936-f2.eps}{Low-resolution spectrum of the discovered new
eclipsing star GSC 3671.0099. The insert highlights the wavelength range
adopted by Yamashita et al. (1977) for spectral classification.}
\IBVSfigKey{5936-f2.eps}{GSC 3671.0099}{spectrum}

\IBVSfig{10.5cm}{5936-f3.eps}{Observed H$_{\alpha}$ profiles and heliocentric 
radial velocities for GSC 3671.0099.}
\IBVSfigKey{5936-f3.eps}{GSC 3671.0099}{radial velocity curve}

%\vskip -0.9 cm

The presence of equally deep primary and secondary eclipses led to expect a
double-lined nature for the newly discovered eclipsing binary.  To verify
this, three medium resolution spectra were obtained with the B\&C
spectrograph mounted on 1.22m Asiago~telescope, and equipped with an ANDOR
iDus 440A CCD camera, housing a EEV~42-10BU back-illuminated chip, 2048$\times$512 pixels of 13.5 $\mu$m size.  A 1200 ln/mm grating provided a dispersion
of 0.61~\AA/pix and a covered wavelength range of 5640-6860 \AA.  On these
spectra, only H$\alpha$ turned out to be strong enough to allow a meaningful
profile to be recorded.  The H$\alpha$ profiles for the three spectra are
presented in Figure~3, were heliocentric radial velocities and orbital
phases are also provided.  Figure~3 gives three strong indications: the
system is double-lined as expected, the velocity separation of the two
components support a mass ratio very close to 1.0, the luminosities of the
two components are similar. However, given the low signal-to-noise ratio of the three
spectra we avoid to use them to improve the orbital solution.

The WD98k93d code (Wilson and Devinney, 1971) was used to obtain only a
photometric solution of the new eclipsing binary.  From the B5V spectral
classification and the calibrations of Strai\v zys and Kuriliene (1981), we
adopted as starting values for the primary a temperature T$_{\rm
eff}$=15400~K and a mass M=4.79 M$_{\odot}$.  The linear limb darkening
coefficients were taken from Van Hamme (1993).  The best fit to the
light-curves was found for a mass ratio $q=0.7$ and
individual masses of M$_1$=5.62 M$_{\odot}$ and M$_2$=3.93 M$_{\odot}$.
These nicely bracket the M=4.79 M$_{\odot}$ corresponding to B5V the spectral
classification and are in visual good agreement with the radial velocity
curves showed in Figure.~4. The semi-major axis is $a=a_1+a_2$=23.62 R$_{\odot}$.  The
orbital inclination is not a critical parameter for deeply eclipsing binary
stars.  We obtained a set of different solutions for different values of the
inclination and we found that $i$= 87.5 was the one best fitting the
observed data.  Finally, we assumed a periastron-syncronized rotation, with
gravity brightening and albedo values equal to 1.  The photometric orbital
solution is given in Tab.~2.  The radii of the stars are similar and close
to the R=3.16 R$_{\odot}$ radius listed by Strai\v zys and Kuriliene (1981)
as typical for B5V stars.  They are far smaller than their Roche lobe radii,
and both component stars are not deformed by binary interaction.  The
light-curves corresponding to the orbital solution are overplotted to the
$B$ and $V$ data in Figure~1 and show an excellent match.

The radial velocity curves corresponding to the photometric orbital solution
of Table~2 are presented in Figure~4, where the measured radial velocities
from Figure~3 are overplotted.  The spectrum for Sept 22, 2009, secured at
photometric orbital phase 0.950, corresponds to passage at stellar
conjunction, and thus fixed to $-$101~km/sec the barycentric velocity of
GSC 3671.0099.  The model radial velocity curves were scaled to this value. 
The radial velocities at the other two observing dates well match the
computed radial velocity curves.  Given the limited resolution and S/N of
the spectroscopic observations, and the fact that we were able to measure
just one line (H$\alpha$), we do not attach more significance to this match
than that of a welcome support to the photometric orbital solution.  More
accurate and far more numerous radial velocities, well distributed in
orbital phase, are necessary to justify a combined photometric/orbital
solution for this binary.  The acquisition of the necessary spectroscopic
observations are encouraged.

\IBVSfig{4.5cm}{5936-f4.eps}{Radial velocity curves predicted from the orbital
solution compared to observations of GSC 3671.0099 from Figure~3 (filled
circles: primary component, stars: secondary component).  The dashed lines
marked the phases of eclipses from Figure~1.}
\IBVSfigKey{5936-f4.eps}{GSC 3671.0099}{radial velocity curve}

\vskip -1.7 cm

\references 

Fitzgerald, M.P., 1970, {\it A\&A}, {\bf 4}, 234

Henden, A., Munari, U., 2000, {\it A\&AS}, {\bf 143}, 343

Henden, A., Munari, U., 2001, {\it A\&A}, {\bf 372}, 145

Henden, A., Munari, U., 2006, {\it A\&A}, {\bf 458}, 339

Landolt, A. U., 1992, {\it AJ}, {\bf 104}, 340 

Miko{\l}ajewska, J., Kenyon, S. J., 1992, {\it AJ}, {\bf 103}, 579

Skopal, A. et al., 2001, {\it A\&A}, {\bf 367}, 199

Strai\v zys, V., Kuriliene, G., 1981, {\it Ap\&SS}, {\bf 80}, 353

Van Hamme, W., 1993, {\it AJ}, {\bf 106}, 2096

Wilson, R. E., Devinney, E. J., 1971, {\it ApJ}, {\bf 166}, 605

Yamashita, Y. et al., 1977, {\it An Atlas of Representative Stellar Spectra}, Univ. Tokyo Press

\endreferences 

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