
\documentstyle[twoside, epsf]{article}

\input ibvs2.sty

\begin{document}
\IBVShead{4536}{8 December 1997}

\IBVStitle{The variability type and period of HD\,143213}


\IBVSauth{U.~BASTIAN$^1$, E.~BORN$^2$}

\IBVSinst{Astronomisches Rechen-Institut, M\"onchhofstr.~14,
D-69120 Heidelberg, Germany}
\vskip -1mm
\IBVSinsto{e-mail: s01@ix.urz.uni-heidelberg.de}
\IBVSinst{Kitzinger Str. 21, D-91056 Erlangen, Germany}

\SIMBADobj{HD143213}
\IBVStyp{ EA }
\IBVSkey{eclipsing binaries}
\IBVSabs{The variability of HD\,143213 was discovered by the TYCHO}
\IBVSabs{instrument of the HIPPARCOS satellite. Here we report on the}
\IBVSabs{classification of GSC 353-301 as an Algol-type variable with a}
\IBVSabs{period of 3.4500 days and an asymmetric secondary minimum.}

\begintext

The variability of HD\,143213~= SAO~121294~= GSC~353-301 was
discovered by the TYCHO
instrument of the HIPPARCOS satellite (Makarov et al., 1994).
Among the 103 usable measurements in the B$_T$ and V$_T$ photometric
channels there were a few discordant
(fainter) ones which indicated a possible eclipsing binary.
The small number and unsuitable temporal distribution of them
made any more definite statement impossible.

E. Born therefore made 310 visual magnitude estimates between
June 1996 and August 1997, using 10$\times$70 binoculars.
This resulted in the classification
of HD~143213 as an Algol-type variable with a period of
3.4500~days ($\pm$0.0003~days). The lightcurve
constructed from Born's observations is displayed in Fig.~1. It is
an Algol-type lightcurve with an eccentric secondary minimum.
The primary and secondary
minima are about 0.5~mag and 0.3~mag deep, respectively.
The width of the minima is about $D=0.2$~days. The secondary
minimum is located
at phase 0.545~($\pm$0.006).
The period was computed from 3 individual primary minima which
were sufficiently well covered to determine precise timings,
a conservative estimate of the uncertainty being 0.02 days in each case.
The temporal distribution of the
310 observations is such that any period other than 3.45~days can be safely
excluded. The ephemeris for primary minima is: \\
\centerline {
     JD(min) = 2450304.35 ($\pm$0.02) + 3.4500 ($\pm$0.0003) $\times$ E
            }

Using this information, the TYCHO measurements were folded with the
now known period. This resulted in the lightcurve shown in Fig.~2.
It is a curious coincidence that the random scatters of the visual
and the satellite measurements are almost the same.
Both the character and the parameters of the variability derived from
the visual estimates are fully confirmed by the TYCHO data. In particular,
the relative phase of the secondary minimum (0.550 $\pm$0.007) is
identical.
Unfortunately, there are only~3 measurements
within a single primary minimum (made within 20 minutes), and~6
measurements within two
different secondary minima.  Obviously, this temporal distribution
of the TYCHO measurements could not have allowed to derive a period.
Also, it is not possible to derive the widths or precise locations
of the minima from the TYCHO data. Nevertheless, the time difference
between the two sets of secondary minimum points (103.99 periods, see
Table~1) confirms the period to within 0.0002~days.

Despite the perfect agreement between the period and shape of the two
lightcurves, there is a strong discrepancy in the phase. Usually,
such a phase shift between two observational sets separated by
a long time interval can be used to improve the period. However,
in the particular case of HD~143213 this turned out to be
impossible.
Any modification of the period to force the
primary minima of both Born and TYCHO to be at
about phase zero resulted in a very significant
separation in phase of Born's 3 primary minima
{\it and} of the 2 TYCHO secondary minima. Thus, any
``improved" period was inconsistent with {\it both}
sets of observational data. But, again,
both sets agree on the
period itself. It perfectly fits the two TYCHO
secondary minima, as well as their separation from the
one observed TYCHO primary minimum. And it also
fits all data of Born (unambiguously; no alias
periods are possible). This point is strengthened
by the perfectly identical phase of the secondary
minima in the two independent sets.

\IBVSfig{3.5cm}{4536-f1.ps}{
Visual light curve of HD 143213; 310 estimates folded with the
period of 3.4500 days.}

\IBVSfig{4.2cm}{4536-f2.ps}{
TYCHO lightcurve of HD 143213; 103 V$_T$ measurements folded with the
same period. Note the phase shift of about 0.59.}

The second problem is that it is not
obvious {\it which} period should be used, since the
TYCHO minimum is at phase 0.59. So, should a
period modification shift it to phase 0.0 or
to phase 1.0? In other words, should a slight increase
or a slight decrease of the period be applied?
There is no way to decide which is better
(actually, both are equally bad, as explained above).
Thus, in addition to contradicting the data,
a correspondingly
changed period would be
ambiguous.

These arguments led us to the conclusion
that the discrepancy cannot be resolved with
the existing data.
We decided to publish this intermediate result
rather than to wait for years until the
discrepancy can be solved by us. The publication
will surely ease the case for other observers,
and give them a guide on what to do. To this end,
we give the existing timings in Table 1.

Observations in the visibility periods 
1998 and 1999 or data from
sky patrol plate archives will resolve the discrepancy.
Table 1 will allow to combine such data
with TYCHO and/or Born, and thus to see where the
cause might be. It could be in the observations or
in the star. On one hand,
a real phase shift of about a day within
7 years is somewhat unlikely. But, on the other hand,
the observational data are very clear.
\vskip 5mm
\centerline{Table 1}
\vskip 3mm
\begin{center}
\begin{tabular}{crlccccccccc}
\hline
BJD   &  E\ \ \ &\ \ \ \ \ \ \ \ \ \ \ Comment \\
\hline
2450304.35~ &   0.00 & min. I from vis. obs. ($\pm$ 0.02 d) \\
2450604.50~ &  87.00 & min. I from vis. obs. ($\pm$ 0.02 d) \\
2450649.35~ & 100.00 & min. I from vis. obs. ($\pm$ 0.02 d) \\
\hline
2450606.37~ &   -    & min. II from vis. obs. ($\pm$ 0.03 d) \\
2450637.39~ &   -    & min. II from vis. obs. ($\pm$ 0.03 d) \\
\hline
2448257.072 & $-$593.41 & TYCHO obs. at min. I magnitude \\
2448257.072 & $-$593.41 & TYCHO obs. at min. I magnitude \\
2448257.087 & $-$593.41 & TYCHO obs. possibly on the rising branch \\
\hline
2447907.100 &      -  & TYCHO obs. at min. II magnitude \\
2447907.100 &      -  & TYCHO obs. at min. II magnitude \\
2447907.114 &      -  & TYCHO obs. possibly on the rising branch \\
2447907.114 &      -  & TYCHO obs. possibly on the rising branch \\
2448265.873 &      -  & TYCHO obs. at min. II magnitude \\
2448265.887 &      -  & TYCHO obs. possibly on the rising branch \\
\hline
\end{tabular}
\end{center}


\vskip 0.5cm

\noindent
Acknowledgements: We thank Andreas Wicenec, ESO Garching, for help with
the retrieval of the TYCHO data.
\vskip 1cm

% Original: \noindent Reference:
\reference

Makarov, V., et al., 1994, {\it IBVS}, No.\,4118
% Original: \noindent Makarov, V., et al., 1994, {\it IBVS}, No.\,4118
% Corrected: Mon Aug 11 13:04:22 MET DST 2003


\end{document}


