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\IBVShead{4808}{24 November 1999}

\IBVStitletl{On the Variability of 19 Aurigae}{ as Observed by the Hipparcos Satellite }

\IBVSauth{CHISTYAKOV, Yu.N.$^1$; SOKOLOV, N.A.$^{1,2}$}

\IBVSinst{Central Astronomical Observatory at Pulkovo, St.\ Petersburg 196140, Russia, e-mail: sokolov@gao.spb.ru}

\IBVSinst{St.\ Petersburg State University, St.\ Petersburg, Russia}

\SIMBADobjAlias{19 Aurigae}{HD 34578}
\IBVStyp{ A }
\IBVSkey{photometry}
\IBVSabs{The variability of the star 19~Auriage was established using}
\IBVSabs{the data from the Hipparcos satellite.}
\IBVSabs{The periodogram analysis of the Hp magnitudes showed} 
\IBVSabs{that all data might be fitted using ephemeris:}
\IBVSabs{$$\rm {JD(Hp~min)}=2447972 + 797 \times E.$$}

\begintext

An important aspect of the differential photometry is to use
non-variable comparison and check stars. Many stars used for this
purpose have not been checked for constancy. Fortunately, the high
quality photometric data from the Hipparcos and Tycho Catalogues (ESA
1997) now permits one to make variability tests for stars down to at
least 8th magnitude. If this is done before the observations, it should
reduce the chances of selecting variable comparison and check stars.
Recently, Adelman~(1998a) has used the photometry from the Hipparcos
catalogue in order to verify the constancy of four magnetic CP stars,
the comparison and check stars used for variability studies with the
0.75-m Four College Automated Photometric Telescope. He found a few
comparison and check stars to be variable. The conclusion about
variability is based on the large standard errors and the amplitudes of
these stars as determined by Hipparcos. However, it is debatable that a
star is constant within standard error and amplitude similar to those
of ``normal'' B and A non-variable stars. In this note we will show
that selecting comparison and check stars using the standard error and
amplitude taken from Hipparcos Catalogue are not enough to draw a
conclusion about constancy of these stars.

\IBVSfig{20.3cm}{4808-f1.ps}{ The Hipparcos photometry of 19~Aur (upper panel) and 18~Aur (lower panel) as a function of time. The solid line is the least square fit.}

\vskip-11pt
To investigate the photometric variability of a double lined eclipsing
binary star AR Aurigae, Adelman~(1998b) has used 18~Aurigae (HR~1734,
Sp: A7~V, $m_{\rm v}=6\fmm49$) as comparison star and 19~Aurigae
(HR~1740 = NSV~1925, Sp: A5~II, $m_{\rm v}=5\fmm03$) as check star.
According to investigation of Adelman~(1998a), 18~Aur and 19~Aur are
non-variable stars, because the standard error and amplitude taken from
Hipparcos Catalogue for both stars is equal to 0\fmm0010 and 0\fmm02,
respectively. However, 19~Aur is marked in SIMBAD data base as
variable, because this star was included in the New Catalogue of
Suspected Variable Stars (Kukarkin et al.~1982). On the other hand,
18~Aur is not marked in SIMBAD data base as variable. We decided to
investigate 19~Aur for variability, before starting the program of the
differential photometry of AR~Aur. Therefore, the Hipparcos Variability
Annex was used, which is available on the world-wide web under
http://astro.estec.esa.hl/Hipparcos/. The Hipparcos Catalogue contains
75 observations of this star made on 21 separate days. It should be
mentioned that the Hipparcos magnitude (Hp) system is close to but
somewhat different from $V$ magnitudes (Van Leeuwen et al.~1997). The
upper panel of Fig.~1 shows the Hipparcos photometry of 19~Aur as a
function of time. As one can see from Fig.~1, the Hp magnitude of the
star is variable. There is a clear maximum around JD~2\,448\,390. In
between, two minima are indicated around JD~2\,447\,980 and
JD~2\,448\,770. To check the observed variation of Hp magnitude of
19~Aur, the star 18~Aur was used. The Hipparcos Catalogue contains 83
observations of this star. It should be noted that 18~Aur was observed
by Hipparcos satellite simultaneously with 19~Aur. The Hp magnitudes of
18~Aur as a function of time are shown in the lower panel of Fig.~1.
The star is non-variable on the span of the observations and it
supports the reliability of the obtained variability of 19~Aur. The periodogram analysis (Scargle 1982) of the Hp magnitudes of 19~Aur showed that the following preliminary ephemeris can be derived:
\begin{equation}
{\rm JD(Hp~min)} = 2447972 + 797 \times E.
\end{equation}
In order to estimate the amplitude of the Hp magnitude variation a
linearized least-squares method was used, which was described by
North~(1987). A least-squares fit by one-frequency cosine curve was
applied to the Hp magnitudes of 19~Aur. The fitted cosine curve was
computed from Eq.~(1) and it was plotted as the solid line on the upper
panel of Fig.~1. The computations give the following results: the
minimum and the maximum values of the Hp magnitude are equal to
5\fmm1224 and 5\fmm1100, respectively. The amplitude is close to the
standard error taken from Hipparcos Catalogue. According to the SIMBAD
data base, the minimum and the maximum values in the $V$ magnitude is
equal to 5.03 and 5.09, respectively. Nevertheless, the span of the
observations covers just over one cycle of the proposed period. Since
no single cycle has been observed over a sufficient fraction of the
light curve, the amplitude and period may be quite spurious. Evidently
19~Aur must be observed in the future in order to make final conclusion about the amplitude and period of its brightness variation.

Finally, the analysis performed in this note has shown that selecting
comparison and check stars using the standard error and amplitude taken
from Hipparcos Catalogue is not without problems. Fortunately, the star
19~Aur was used as the check star by Adelman~(1988b), but it should be
replaced in future studies of AR Aurigae.

This work is partly supported by the Russian National Foundation for
Astronomy (project No.\ 1.4.1.2). The research has been made use of the
SIMBAD data base, operated by the CDS at Strasbourg, France.

\references

Adelman, S.J., 1998a, {\it Baltic Astronomy}, {\bf 7}, 427 

Adelman, S.J., 1998b, {\it Astron.\ Astrophys.\ Suppl.\ Ser.}, {\bf 128}, 245

ESA, 1997, The Hipparcos and Tycho Catalogues, SP-1200

Kukarkin, B. V., Kholopov, P. N., Artiukhina, N. M., et al., 1982, 
New Catalogue of Suspected Variable Stars, Moscow, Nauka Publishing Office

North, P., 1987, {\it Astron. Astrophys.\ Suppl.\ Ser.}, {\bf 69}, 371

Van Leeuwen, F., Evans, D. W., Grenon, M., et al., 1997,
{\it Astron. Astrophys.}, {\bf 323}, L61
 
Scargle, J. D., 1982, {\it Astrophys.\ J.}, {\bf 263}, 835 

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