
\documentstyle[epsf]{article}

\input ibvs.sty

\begin{document}

% Call for macro \IBVShead{Number}{Date} - To be filled by the editors!
\IBVShead{4124}{13 December 1994}     % Use it with dummy or empty arguments

% Call for macro \title {Title of the paper}
\centerline{\ft GENEVA PHOTOMETRY OF THE ECLIPSING BINARY TV Nor}
\vskip 8mm

% Call for macro \begintext
\begintext

The eclipsing binary HD 143654 = TV Nor has been brought to the attention
of spectroscopists by Renson (1990) because it has been classified
Ap EuCrSr by Houk (1978). Indeed, there is as yet no clear-cut case of an
Ap star of the Si or SrCrEu type belonging to an eclipsing system. This is
probably because of the lack of short period binaries among this kind of
peculiar stars (see e.g. Gerbaldi et al. 1985). However, such a system
would be most interesting, because it is known that the abundance anomalies
are not evenly distributed on the surface of the star, but are concentrated
into patches following the large-scale magnetic structure. Many Ap stars
are therefore spectrum variables simply because their rotation brings
these patches into view and away from it; the art of ``Doppler imaging''
exploits the resulting line variations to map the abundance anomalies on
the surface of the star. Such maps, however, present ambiguities, especially
regarding the latitudes of the patches. Observing an Ap star undergoing an
eclipse by a normal, constant companion would help to remove the ambiguities,
and even to build an independant map of the eclipsed hemisphere for the
corresponding rotational phase (Piskunov \& Rice, 1993; Vincent et al., 1993).

In the hope that HD 143654 could be such a rare system, we monitored it with
Geneva photometry in order to obtain a good quality lightcurve.
All measurements have been made from the European Southern Observatory,
La Silla, Chile, with the double-beam ``P7'' photometer attached
to the 0.7m Swiss telescope. The first
three, routine measurements were made as early as 1982, 1983 and 1984
respectively, and 5 additional measurements were made in April 1989 by
the late Dr. Zdenek Kviz. Systematic monitoring began essentially in June 1990,
stimulated by Renson's IBVS note, and continued each following season until
July 1994.

We expected that this well-detached system would present periodic variations
outside the eclipses, due to the intrinsic variations of the Ap component.
The result is, however, disappointing from this point of view, as shown
in Figure 1 which displays the [U-B], [B-V] and V curves. Outside the eclipses,
the r.m.s. standard deviations are respectively 0.0090, 0.0049 and 0.0047,
which can be entirely attributed to measurement uncertainties. Thus, neither
the primary nor the secondary is photometrically variable in a significant
way, making any spectral variation rather improbable.

It is interesting to notice that the minima have different depths in V and
vary in opposite ways in [B-V], while [U-B] remains constant. This means
that the components have significantly different effective temperatures
and spectral types. The primary is probably an early or mid- A dwarf while the
secondary may be an A8 or F0 giant. This fact is probably significant
regarding the Ap classification: the spectrum appears composite, and was
very likely misinterpreted as that of a single Ap EuCrSr star.
\vfill \eject

\centerline{2}
\vskip 5mm
\epsfysize=200mm
\centerline{\epsffile{4124-f1.ps}}
\vskip 5mm
\centerline{Figure 1. Geneva [U-B], [B-V] and V lightcurves of the eclipsing
binary TV Nor,}
\centerline{folded according to the period of Hertzsprung (1937). Notice the
lack of}
\centerline{variability between the eclipses.}
\vfill \eject

\centerline{3}
\vskip 2mm
This is confirmed by the behaviour of the peculiarity parameter
$\Delta (V1-G)$ as a function of phase: while this parameter is generally
positive for Ap stars (see e.g. Hauck \& North 1982), it remains here at
the constant value
\vskip 1mm
\centerline{$\Delta (V1-G) = -0.006$}
\vskip 1mm
\noindent with an r.m.s. residual scatter of only 0.0055 magnitudes. There is no
significant variation at all during the eclipses. Therefore, the
$\Delta (V1-G)$ value of not only the whole system, but also of each component
of TV Nor is quite typical of normal stars.

Furthermore, one of us (PN) has taken a spectrum of this system with the 1.4m
CAT telescope of the European Southern Observatory, equipped with the CES
spectrograph, the Long Camera and the FA 2048 CCD detector (ESO CCD \# 30).
The resolving power was R = 60000. The spectrum was taken in the $H_\alpha$
region on the night of May 16-17, 1994 at HJD = 2449489.607,
i.e. at phase $\phi = 0.2531$ according to the ephemeris given in Figure 1.
This is practically at a quadrature and allows us to estimate the total
mass of the system from the relative velocity at that phase and from the
Kepler's third law. Fitting a gaussian to the core of the $H_\alpha$ line
of each component, we obtain $\lambda_p = 6560.984 {\rm \AA}$ and
$\lambda_s = 6564.447 {\rm \AA}$, implying a relative velocity
$\Delta V_r = 158.2 ~{\rm km~s^{-1}}$. If the orbit is circular,
which is very probably the case
since the eclipses have the same duration and are separated by exactly
half a period, and if the inclination $i$ of the orbital plane is close to
$90^{\rm o}$ as the very existence of deep eclipses suggests, then we obtain
for the semi-major axis
\epsfysize=125mm
\centerline{\epsffile{4124-f2.ps}}
\centerline{Figure 2. High-resolution spectrum of TV Nor taken at orbital
phase 0.253,}
\centerline{i.e. at quadrature, and showing the star as a double-lined system.}
\vfill \eject

\centerline{4}
\vskip 2mm
\centerline{a = 0.124 AU}
\vskip 1mm
\noindent and for the total mass
\vskip 1mm
\centerline{$M_p + M_s = 3.50 M_\odot$}
\vskip 1mm
\noindent which is quite consistent with the spectral types and luminosity
classes proposed above for the two components.

This binary may well be spectroscopically normal, which would designate it
as a very good candidate for precise mass and radius determination.

Acknowledgements: This work was supported in part by the Fonds National
de la Recherche Scientifique. Drs. Z. Kviz, P. Lampens and Mrs. C. Nitschelm
and F. Barblan contributed to the photometric observations, which were
reduced by Mr. C. Richard.
\vskip 5mm
% Call for macros \authors - \authorsrightcol - \endauthors
\beginauthors
\authorsrightcol
P. NORTH
Institut d'Astronomie de l'Universit\'e
de Lausanne
CH-1290 Chavannes-des-Bois,
Switzerland
\vskip 2mm

M. BURNET
Observatoire de Gen\`eve
CH-1290 Sauverny, Switzerland
\endauthors

% Call for macro \references
\references

Gerbaldi, M., Floquet, M., Hauck, B., 1985, {\it A\&A} {\bf 146}, 341

Hauck, B., North, P., 1982, {\it A\&A} {\bf 114}, 23

Hertzsprung, E., 1937, {\it Bull. Astr. Netherlands} {\bf 8}, 168

Houk, N., 1978, {\it Michigan Catalogue of two-dimensional spectral Types
for the HD Stars}, Vol. 2

Piskunov, N.E., Rice, J.B., 1993, {\it PASP} {\bf 105}, 1415

Renson, P., 1990, {\it IBVS}, No. 3452

Vincent, A., Piskunov, N.E., Tuominen, I., 1993, {\it A\&A} {\bf 278}, 523

\end{document}
