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\begin{document}
\IBVShead{4999}{7 December 2000}
\IBVStitle{Delta Velorum is an Eclipsing Binary}

\IBVSauth{Otero, Sebastian A.$^1$; Fieseler, Paul D.$^2$; Lloyd, Christopher$^3$}
\IBVSinst{Liga Iberoamericana de Astronomia (LIADA), e-mail: {\tt varsao@fullzero.com.ar}}
\IBVSinst{Galileo Mission, Jet Propulsion Laboratory, NASA, CalTech,
e-mail: {\tt fieseler@mail1.jpl.nasa.gov}}
\IBVSinst{Space Science \& Technology Department, Rutherford Appleton
Laboratory, Oxfordshire, UK,\\ \indent e-mail: {\tt cl@astro1.bnsc.rl.ac.uk}}

\SIMBADobjAlias{Delta Vel}{HD 74956}
\IBVStyp{ EA }
\IBVSkey{variable stars, eclipsing binary, periods}
\IBVSabs{From visual and Galileo star scanner observations the principal}
\IBVSabs{component of the bright multiple star delta Velorum is found to be}
\IBVSabs{an eclipsing binary, in an eccentric orbit, with a period of 45.150}
\IBVSabs{days. The system probably contains two early A-type stars.}

\begintext

Delta Velorum (HR 3485, HD 74956, HIP 42913) has been known as a
quadruple system for many years as IDS 08419--5420 (Jeffers et al.,
1963). The system contains two common proper motion (CPM) pairs, AB
(2\arcs) and CD (6\arcs), separated by $\sim 69\arcs$. The brighter
pair, $\delta$ Vel (A1V, $V=1.96$, $B-V=+0.04$, according to Simbad)
comprises a bright A-type star and a companion of unknown spectral
type, three magnitudes fainter.  The primary component was itself
resolved as a double star by speckle interferometry (0\farcs6) in 1978
(Tango et al., 1979) and then by the Hipparcos satellite (0\farcs736,
$\Delta H_{\rm p}=3.58$, Perryman et al., 1997), although this
knowledge has not made its way to all the catalogues. The CCDM
(Dommanget \& Nys, 1994) and the MSC (Tokovinin, 1997) only mention the
four well known components. The WDSC (Worley \& Douglass, 1996), the
Third Catalogue of Interferometric Measurements of Binary Stars
(Hartkopf et al., 2000) and the GCPD (Mermilliod et al., 1997) do
include the sub-arcsecond component.

\begin{table}[bp]
\begin{center}
\caption{Times of minima}
\begin{tabular}{lrrcrl} \hline
\multicolumn1c{JD} & Cycle & $O-C$ & Min & Mag. & Comment \\ \hline
2447851.692 & 0 & 0.00 & II & 2.2 &UB Galileo \\
2450631.5 & 62 & 0.10 & I & 2.2 &vis Otero \\
2450831.7 & 66 & 0.10 & II & 2.2 &vis Otero \\
2451147.8 & 73 & 0.15 & II & 2.1 &vis Otero \\
2451308.65 & 77 & 0.00 & I & 2.3 &vis Otero \\
2451715.1 & 86 & 0.10 & I & $>2.1$ &UB Galileo \\
2451850.45 & 89 & 0.00 & I & 2.4 &vis Jansen \\
2451869.8 & 89 &$-$0.24 & II & 2.1 &vis various \\ \hline
\end{tabular}
\end{center}
\end{table}

On July 1, 1997, a drop of $\sim 0\fmm3$ in $\delta$ Vel's brightness
was observed visually by Otero (2000a), who went on to observe three more
fadings during 1998 and 1999. Independently, Fieseler (2000) reported a
fading in the engineering data of the Galileo star scanner observations
of $\delta$ Vel, on June 19, 2000. Examination of the only
other set of Galileo observations of $\delta$ Vel, which was made in
1989, shows a particularly well observed eclipse (see Figure~1).
Hipparcos also observed $\delta$ Vel but apart from one observation of
poor reliability the 110 observations, made fairly evenly over $\sim 1200$
days, show that $\delta$ Vel was constant to within $\sim 0\fmm01$ at
these times. 

From an analysis of all the observations available, Lloyd (2000) and
Otero (2000b) independently were able to fit an eccentric orbit with a
period of approximately 45.16 days, which was consistent with both the
eclipses and the out-of-eclipse observations, principally from Hipparcos.
An international observing campaign across the southern hemisphere using
this ephemeris resulted in the visual detection of eclipses on November
1st and 21st, 2000, near the predicted times. The times of minima from all the
eclipses are gathered in Table 1 and were used to derive the improved
ephemeris,
$$\begin{array}{l}
{\rm JD_I} = 2447832.10\mathbin{(\pm}0.05) + 45.150\mathbin{(\pm}0.001) \times E,\\
{\rm JD_{II}} = 2447851.692\mathbin{(\pm}0.003) + 45.150\mathbin{(\pm}0.001) \times E.\\
\end{array}$$

The best observed eclipse is that seen by Galileo in 1989 and this is
shown in Figure 1 together with $\alpha$ Leonis for comparison. The
Galileo star scanner is used for spacecraft orientation, and not for
science, so the data are not calibrated. The star scanner uses an
unfiltered photomultiplier tube with a response covering the Johnson $U$
and $B$ bands, peaking at $\sim 4300$~\AA. The light curve shows an
almost complete eclipse, with a full width of 1.0 days, and a depth of
0\fmm30.

\IBVSfig{8cm}{4999-f1.eps}{The light curve around secondary eclipse
observed by the Galileo star tracker. The magnitudes are in the natural
system of the detector normalised to the $V$ magnitude of $\delta$
Vel. Observations of $\alpha$ Leonis are shown for comparison}

The 2000 Galileo observations have poorer coverage than the 1989 data
as Galileo's telemetry rates were lower, because the spacecraft was
further from the Earth. Also software was introduced to suppress
discordant measurements, which means that observations below $\sim
2\fmm15$ are not reported by the spacecraft. Unfortunately this makes
it very difficult to compare the two eclipses and assess the relative
depths. The other eclipses observed visually do show a marginal but
systematic difference. The three secondary eclipses typically reach
2.1 or 2.2, which is consistent with the 1989 Galileo observations,
while the three primary eclipses observed were all deeper than 2.2,
probably reaching 2.3. There is also some suggestion in the visual
observations that the primary eclipse is shorter than the secondary. As
the orbit is eccentric, secondary eclipse occurs at $\phi = 0.43$, and
the period quite long, it is possible that the system is elliptical
enough to show differences in the width of the eclipses.

Given that there are three resolved components of $\delta$ Vel it may
be considered that there is a possible uncertainty in the
identification of the star responsible for the variations. However, the
variable is unambiguously the brightest component, A\@. The close
companions Aa and B each contribute $\sim 0\fmm05$ to the total light
of the system so they cannot be responsible for the observed level of
variation.

In the absence of any other information it would appear that both stars
in the eclipsing binary are relatively similar. Any substantial
difference would reveal itself in the eclipses, and this appears to be
small. Without an accurate description of the primary eclipse, an
orbital solution and detailed modelling, it is impossible to draw any
definitive conclusion. However, if the difference between the eclipses
is only $\sim 0\fmm1$ then this points to only a small difference in
temperature and radius between the two components. The spectral type,
given variously as A0V or A1V, is probably composite, and the system is
probably a double-lined spectroscopic binary, with $K >
50$~km\,s$^{-1}$. The Bright Star Catalogue notes that the velocity is
possibly variable, and quotes two rather different values of the $v\sin
i$, 0 and 80~km\,s$^{-1}$.

We conclude that $\delta$ Velorum A is an eclipsing binary star probably
containing two early main-sequence A-type stars. The $\delta$ Vel system now
contains six components. The spectroscopic binary in an eccentric orbit,
the close companions Aa and B, and the more distant CPM pair CD.

Further photometric and spectroscopic observations that will help in
modelling the system are encouraged.

A portion of this research was funded by NASA and performed at the Jet
Propulsion Laboratory, California Institute of Technology. We would
like to thank the following observers for contributing their
observations to the international campaign: Albert Jansen and Brian
Fraser (South Africa), Fraser Farrell and Michael Mattiazzo
(Australia), Jaime Garc\'{\i}a and Sergio Dom\'{\i}nguez, (LIADA,
Argentina).

\references

Dommanget, J., Nys, O., 1994, {\it Comm.\ Obs.\ R. Belgique, Ser.\ A}, No.\ 
{\bf 115}, Catalogue of Components of Double and Multiple Stars (CCDM)

Fieseler, P.D., 2000, AAVSO private communication

Hartkopf, W.I., McAlister, H.A., Mason, B.D., 2000, CHARA, Georgia State 
University, Third Catalogue of
Interferometric Measurements of Binary Stars

Jeffers, H.M., van Den Bos, W.H., Greeby, F.M., 1963, {\it Publ.\ Lick Obs.}, 
{\bf 21} Index catalogue of visual double stars, 1961.0

Lloyd, C., 2000, vsnet-campaign-deltavel No.\ 4 ({\tt
http://www.kusastro.kyoto-u.ac.jp/ vsnet/Mail/vsnet-campaign-deltavel/msg00004.html})

Mermilliod, J.-C., Hauck, B., Mermilliod, M., 1997, {\it A\&AS}, {\bf 124}, 349,
General Catalogue of Photometric Data (GCPD) II

Otero, S., 2000a, {\tt http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet-newvar/ msg00599.html}

Otero, S., 2000b, vsnet-campaign-deltavel No.\ 3 ({\tt
http://www.kusastro.kyoto-u.ac.jp/ vsnet/Mail/vsnet-campaign-deltavel/msg00003.html})

Perryman, M.A.C., et al., 1997, The Hipparcos and Tycho Catalogues, ESA
SP-1200

Tango, W.J., Davis, J., Thompson, R.J., Hanbury Brown, R., 1979, {\it
Proc.\ Astron.\ Soc.\ Australia}, {\bf 3}, 323

Tokovinin, A.A., 1997, {\it A\&AS}, {\bf 124}, 75, Multiple Star
Catalogue 1997--1999 (MSC), CDS III/191 1999 revision

Worley, C.E., Douglas, G.G., 1997, {\it A\&AS}, {\bf 125}, 523,
 The Washington Visual Double Star Catalogue, 1996.0 (WDS)

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