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\IBVShead{6066}{3 August 2013}

\IBVStitletl{Period changes in the eclipsing binary DX V\lowercase{el}}


\IBVSauth{Volkov, I.M.$^{1,2}$; Chochol, D.$^2$; Grygar, J.$^3$; Jel\'inek, M.$^4$; Kub\'anek, P.$^3$; Ma\v{s}ek, M.$^3$; Prouza, M.$^3$; Ribeiro, T.$^5$; Sebastian, D.$^6$; Van Houten, C.J.$^7$}

\IBVSinst{Sternberg Astronomical Institute, Moscow University,Universitetsky Ave. 13, Moscow 119992, Russia \newline E-mail: {\tt imv@sai.msu.ru}}

\IBVSinst{Astronomical Institute of the Slovak Academy of Sciences, 059 60 Tatransk\'a Lomnica, Slovakia \newline E-mail: {\tt chochol@ta3.sk}}

\IBVSinst{Institute of Physics Czech Academy of Sciences, 182 21 Praha, The Czech Republic \newline E-mail: {\tt grygar@fzu.cz}}

\IBVSinst{Instituto de Astrof\'isica de Andaluc\'ia (CSIC), Glorieta de la Astronom\'ia, s/n, 18008 Granada, Spain }

\IBVSinst{Institution: SOAR Telescope, Colina El Pino s/n - Casilla 603 - La Serena, Chile \newline E-mail: {\tt tribeiro@ctio.noao.edu}}

\IBVSinst{Thuringer Landessternwarte Tautenburg, Sternwarte 5,D-07778 Tautenburg, Germany \newline E-mail: {\tt sebastian@tls-tautenburg.de}}

\IBVSinst{Sterrewacht, Huygens Laboratorium, 2300 RA Leiden, The Netherlands (passed away August 24, 2002)}


\SIMBADobjAliasJ2000{DX Vel}{GSC 8193-02704}{09 51 45.98 -50 53 23.7} 
\IBVStyp{  EA/SD:  } 
\IBVSkey{photometry}

\IBVSabs{Comparison of the old and new photometric observations of the}
\IBVSabs{eclipsing binary DX~Vel revealed the long term orbital period}
\IBVSabs{decrease, caused either by the mass transfer/mass loss effects}
\IBVSabs{or the presence of the third component in the system.}

\begintext
The eclipsing binary DX~Vel (HD~297655, $P$\ = 1.12 days,
$V$~=~10.2, A5, detached Algol binary) was discovered and studied
photographically by van Houten (1950). In 1978 van Houten obtained the precise
photoelectric light curve of the object in the  Walraven system.
The observations were published by van Houten et al. (2009).

Our CCD photometry of DX~Vel was obtained between May 2012 and January
2013, using the FRAM (Photometric Robotic Atmospheric Monitor):
0.3\,m Schmidt-Cassegrain telescope located in Argentina near the
town of Malargue and operated by Institute of Physics, Czech
Academy of Sciences (for more details see Prouza et al., 2010).
The telescope FRAM works in fully automatic mode using RTS2
software system (Kub\'{a}nek et al., 2004) as a part of the
atmospheric monitoring program of the Pierre Auger Observatory
(for the description of the Pierre Auger Observatory see e.g.
Abraham et al., 2010). The telescope is equipped with the CCD
camera G2-1600 of Moravian Instruments and Bessel set of filters.
The observations were acquired in the standard $BVRcIc$ system.

The light curve analysis of our data with the PHOEBE software (Prsa \&
Zwitter 2005), assuming the sphere-sphere model with reflection
effect subtracted, provides the parameters given in
Table~1. The corresponding fit of the $R$ light curve is shown in
Fig.~1. The solution allows as much as 12 percent of third
light in the system. We obtained the same results from van Houten's
and ASAS light curves of DX~Vel. As seen in Fig.~1, Van Houten's
observations show small but significant anomalies in colour
changes in the primary minimum. The source of the extra $U$-light
in the bottom of the primary minimum could be an F star object,
the third body in the system.

\begin{table}[!hb]
\centerline{{\bf Table 1.} Parameters of DX~Vel}
\begin{center}
\begin{tabular}{cc}
\hline
$r_1$ & $0.333\pm 0.002$ \\
$T_1$ & $7400\pm 300K$ \\
$r_2$ & $0.236\pm 0.003$ \\
$T_2$ & $4400\pm 400$ \\
$i$   & $85.4-89.9$   * \\
\hline
\end{tabular}
\end{center}
\begin{center}
* depends on the value of the third light L3, varies from 0 to
0.12 in $V$ and a little bit less in red and infrared.
\end{center}
\end{table}


\IBVSfig{11cm}{6066-f1.eps}{Part of our $Rc$ light curve of DX~Vel
around both minima, taken by FRAM. Points denote the individual
observations, solid red line stands for the theoretical fit,
according to the parameters from Table~1 with L3=0.06. Van Houten's
data transformed into $U-B$ and $B-V$ colours and phased with the
same formula as $Rc$ points are shown in two bottom panels.}
\IBVSfigKey{6066-f1.eps}{DX Vel}{O-C diagram}

In May 2012, Tiago Ribeiro obtained a spectrum of DX~Vel
during its primary minimum, with the 4-m SOAR telescope, using the
GOODMAN spectrograph with the 1200 l/mm grating and the 0.46" slit
allowing the spectral resolution of $R$=5900. The spectral classification
was done by Daniel Sebastian, who classified the primary component of
DX~Vel as an A5 IV-III object. The classification was done by an automatic
comparison of the spectrum with a catalog of template-spectra provided by
Valdes et al. (2004). The used method is described in detail in
Sebastian et al. (2012). The comparison of the part of our spectrum
with the A5 III template is shown in Fig.~2. The size of the binary system with
the orbital period 1.12 days is too small to contain an A5 subgiant or giant.
The easiest explanation is to assume a third body in the DX~Vel system,
which affects the spectral classification.

\IBVSfig{11cm}{6066-f2.eps}{Comparison of the spectrum of DX~Vel (green line)
with the A5 III template (red line).}
\IBVSfigKey{6066-f2.eps}{DX Vel}{spectrum}

All available primary minima times were used to construct the eclipse
time variation (ETV) diagram. To derive the precise times of minima
from photoelectric and CCD observations, we fitted the
synthetic light curve through the individual observations by means
of the least squares method. For simultaneous observations in
several filters the timings of minima were weighted and mean
values were calculated. To study the long-term period changes, we
used 8 photographic minima times of DX~Vel, published by van
Houten (1950) and Hoffmeister (1951) to calculate one normal epoch
of the primary minimum with a suitable precision. The ASAS data
were divided into 6 intervals and normal minima for each interval
were obtained. The times of minima are listed in Table~2. The
$O-C$ residuals in the ETV diagram (Fig.~3) were calculated using the
linear ephemeris: HJD Min~I = 2443583.4618 + $1\fday11730385\times$E,
obtained by the least squares method.

\begin{table}[!hb]
\centerline{{\bf Table 2.} Times of minima for DX~Vel}
\begin{center}
\begin{tabular}{lrcl}
\hline
HJD $-$ 2400000 & Cycle & Residual (days) & Remarks \\
\hline
29037.280(8)    &$-$13019 & $-$0.0030  & photographic \\
43583.4660(2)   &      0  & $+$0.0042  & van Houten \\
51985.5894(7)   &   7520  & $+$0.0027  & ASAS \\
52578.8771(7)   &   8051  & $+$0.0020  & ASAS \\
53165.4619(6)   &   8576  & $+$0.0023  & ASAS \\
53870.4778(8)   &   9207  & $-$0.0006  & ASAS \\
54482.7602(10)  &   9755  & $-$0.0007  & ASAS \\
54967.6704(12)  &  10189  & $-$0.0003  & ASAS \\
56053.6880(5)   &  11161  & $-$0.0021  & our observations \\
56080.5031(6)   &  11185  & $-$0.0023  & our observations \\
56292.7906(4)   &  11375  & $-$0.0025  & our observations \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{8cm} {6066-f3.eps}{The ETV diagram for DX~Vel. Solid red
line - parabolic approximation, dashed line - the third body
solution.}
\IBVSfigKey{6066-f3.eps}{DX Vel}{other}

The minima times in ETV diagram can be fitted either by a parabola
(continuous period change), represented by the following ephemeris:
\begin{center}
HJD ${\rm Min~I~} = 2443583.4660(8) + 1\fday11730390(2)\times E - 5.51(4)\cdot 10^{-11}\times E^2$ \\
\end{center}

\noindent or by the third body orbit. Parameters of the formal third-body fit
found by the least squares method are as follows:

\begin{center}
$P_3~{\rm(period)} = 28500\pm 500$ days, i.e. 78.0 years \\
$T_0~{\rm(time~of~periastron) = J.D.}~2 428 200 \pm 500$ \\
$A~{\rm(semiamplitude)} = 0.00405 \pm 0.00020$ day \\
$e = 0.9 \pm 0.1$ \\
$\omega = 245\deg\pm 15\deg$. \\
\end{center}

We would like to note that the similar situation in colour changes in minima
had resulted in discovery of the third body in V577~Oph system
(Volkov, 1991; Volkov \& Volkova, 2010) and EQ~Boo (Volkov et al.,
2011, 2012).

We can conclude that the revealed orbital period change in DX~Vel
can be either real, caused by the mass transfer/mass loss effects or
apparent, caused by the existence of the third body in the system.

{\bf Acknowledgements}. This study was partly supported by the
scholarship of the Slovak Academic Information Agency, RFBR grant
11-02-01213a and by the VEGA grant No. 2/0094/11. The operation of
the robotic telescope FRAM is supported by the EU grant GLORIA
(No. 283783 in FP7-Capacities program) and by the grant of the
Ministry of Education of the Czech Republic (MSMT-CR LA08016).
This work is partially based on observations obtained at the Southern
Astrophysical Research (SOAR) telescope, which is a joint project
of the Minist\'{e}rio da Ci\^{e}ncia, Tecnologia, e
Inova\c{c}\~{a}o (MCTI) da Rep\'{u}blica Federativa do Brasil, the
U.S. National Optical Astronomy Observatory (NOAO), the University
of North Carolina at Chapel Hill (UNC), and Michigan State
University (MSU).


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\endreferences

\IBVSedata{6066-t3.txt}
\IBVSdataKey{6066-t3.txt}{DX Vel}{photometry}
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\IBVSdataKey{6066-t5.txt}{DX Vel}{photometry}
\IBVSedata{6066-t6.txt}
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