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\IBVShead{5757}{23 February 2007}

\IBVStitletl{FR Scuti: a Triple VV Cephei-type System}{ of Particular Interest}

\IBVSauth{Pigulski, A.; Michalska, G.} 

\IBVSinsto{Instytut Astronomiczny Uniwersytetu Wroc{\l}awskiego, Kopernika 11, 51-622 Wroc{\l}aw, Poland \\
e-mail: pigulski@astro.uni.wroc.pl, michalska@astro.uni.wroc.pl}

\SIMBADobjAliasJ2000{FR Sct}{HIP 90115}{18 23 22.79 12 40 51.8}
\GCVSobj{FR Sct}
\IBVSabs{We analyse ASAS, Hipparcos and other archival photometry of FR Sct}
\IBVSabs{to show it is a hierarchical triple system of VV Cephei type consisting}
\IBVSabs{of a M-type supergiant and close binary of OB stars.}

\begintext

The VV~Cephei-type binaries form a small but interesting group of massive binaries consisting of an M-type supergiant
and a late O or an early B-type star (Bidelman, 1954; Cowley, 1969). They are related to, but distinct from two other
classes of stars with composite spectra: symbiotic stars and $\zeta$~Aurigae systems. The optical spectra of VV~Cephei
stars are characterized by emission lines of hydrogen and [Fe II]. In addition, weaker emission lines, mostly forbidden,
of the other single-ionized elements are observed.  Because of the large radius of the M-type supergiant, the orbital
periods in VV~Cephei systems might be decades long, like for the prototype, VV~Cep (20.4~yr), or KQ~Gem (26.7~yr). In a
few of them, including VV~Cep itself, eclipses are observed. The VV~Cephei systems are very rare; less than twenty are
known in the Galaxy. This rarity comes from the fact that systems with very massive components evolve very fast.

FR~Sct (HIP 90115) is a relatively poorly studied VV~Cephei system. Its composite spectrum was discovered by Bidelman \&
Stephenson (1956). In contrast to the other VV~Cephei systems, it showed emission lines of [Fe III] and [O III]. The
photometric variability was discovered by Shajn (1934).  Although Shajn (1935) noted that the star exhibits variability
with a short period (of unknown length), the observations made so far (Tsessevich, 1952; Burchi, 1980, Hipparcos data)
showed no more than erratic or semi-regular variations in the range of a few tenths of magnitude.

FR Sct is also known as a radio source (Florkowski et al., 1985). The radiation in the radio domain is probably due to a
thermal emission of a cloud of plasma. The plasma originated probably as a result of ioniziation of the cool wind coming
from M supergiant by the ultraviolet radiation of the OB component. The radio and optical positions of FR~Sct were
frequently used to define or compare astrometric reference frames (e.g., Johnston et al., 1985; Walter et al., 1997).

The star was also observed by the ASAS survey (Pojma\'nski, 1997) where it is recognized as ASAS 182323$-$1240.9.  
Surprisingly, automatic classification applied by the authors of the ASAS catalogue to this star resulted in an ESD/ED
classification, i.e., semi-detached or detached eclipsing binary, with a period of only 3.535~d (Pojma\'nski \&
Maciejewski, 2005).  What seemed to be at a first glance an incorrect classification, has been confirmed during our
analysis, carried out according to the procedure described by Pigulski (2005). The only difference was that the search
for periodic variations we present here was made using eclipse-freed light curves. This was a part of a much wider
search for pulsating components of eclipsing binaries (Michalska \& Pigulski, 2007). The original ASAS light curve
(Fig.~1) does not show the eclipses in an obvious way, because they are contaminated by the quasi-periodic variations
originating probably in the M-type supergiant. However, as these long-term variations could be well represented by means
of a series of sinusoidal terms with frequencies smaller than 0.01~d$^{-1}$, we were able to separate them from
eclipses. The contributions from the long-term variations and the eclipses are shown in Figs.~2 and~3, respectively. As
can be seen in Fig.~2, the long-term changes, presumably due to the variability of the cool supergiant, have a range of
about 0.4~mag and a mean $V$ magnitude of about 10.28.  The larger scatter after HJD~2453300 is due to the change of
the exposure time to smaller value around this date in the ASAS observations. In consequence, the mean accuracy of a
single measurement amounts to about 0.02~mag for observations made prior and 0.06~mag for observations made after that
date.

\IBVSfig{6.4cm}{5757-f1.eps}{The $V$-filter ASAS light curve of FR~Sct. The data cover the interval between February
2001 and June 2006. Data plotted as crosses are of lower quality}
\IBVSfigKey{5757-f1.eps}{FR Sct}{light curve}

\IBVSfig{5.7cm}{5757-f2.eps}{The same as in Fig.~1, but freed from the contribution from the eclipses}
\IBVSfigKey{5757-f2.eps}{FR Sct}{light curve}

On the other hand, the eclipse light-curve (Fig.~3) shows two minima of unequal depth; about 0.23~mag for the primary
and 0.13~mag for the secondary eclipse. The epochs of the primary minimum, as derived from the ASAS data, can be
represented by the following ephemeris:
\begin{equation} T_{\rm min\,I} = {\rm HJD}~2452082.802 \pm 0.006 + (3.53405 \pm 0.00004) \times E,
\end{equation}
where $E$ is the number of cycles elapsed from the initial epoch.

\IBVSfig{9.5cm}{5757-f3.eps}{The eclipses in FR~Sct. The light curve was folded with the orbital period of 3.53405~d.  
Like in Figs.~1 and 2, the data obtained prior to HJD~2453300 are plotted with dots, after that date, with crosses. The
data were freed from the long-term changes seen in Fig.~2}
\IBVSfigKey{5757-f3.eps}{FR Sct}{light curve}

The immediate conclusion coming from the length of the orbital period is that the eclipses cannot occur between the hot
component and the cool M-type supergiant. In that case we would expect the orbital period of at least a few years.
Consequently, the most plausible explanation is that the hot component of FR~Sct is itself a binary, and what we see are
the eclipses in this system. Thus, FR~Sct would be a hierarchical triple system consisting of very massive stars. This
makes it a very interesting star for the follow-up study and unique among VV~Cephei stars.

It has to be pointed out that the separation of the eclipsing light curve (Fig.~3) and the long-term changes (Fig.~2) we
made does not mean that Figs.~2 and 3 represent the light changes of the M-type supergiant and the hot binary as if they
were seen separately. First, in both cases the contribution from the other component(s) leads to the reduction of the
amplitude of the light curve. Next, we cannot exclude that some erratic changes seen in Fig.~2 come from the hot
components. The presence of the [Fe III] and [O III] emission lines in the spectra of FR~Sct (Bidelman \& Stephenson,
1956) may be related to the duplicity of the hot component. The other possibility is that the hot components in FR~Sct
are hotter than usually the case in VV~Cephei systems.

{\bf Acknowledgement.} The work was supported by the MNiI grant 1 P03D 016 27.

\references

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Bidelman, W.P., Stephenson, C.B., 1956, {\it PASP}, {\bf 68}, 152

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Cowley, A.P., 1969, {\it PASP}, {\bf 81}, 297

Florkowski, D.R., Johnston, K.J., Wade, C.M., de Vegt, C., 1985, {\it AJ}, {\bf 90}, 2381

Johnston, K.J., de Vegt, C., Florkowski, D.R., Wade, C.M., 1985, {\it AJ}, {\bf 90}, 2390

Michalska, G., Pigulski, A., 2007, {\it Comm.~in Asteroseismology}, in press

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Pojma\'nski, G., 1997, {\it Acta Astron.}, {\bf 47}, 467

Pojma\'nski, G., Maciejewski, G., 2005, {\it Acta Astron.}, {\bf 55}, 97

Shajn, P.T., 1934, {\it Perem.~Zvezdy}, {\bf 4}, 342

Shajn, P.T., 1935, {\it Poulkovo Obs.~Circ.}, {\bf 13}, 30

Tsessevich, V.P., 1952, {\it Perem.~Zvezdy}, {\bf 8}, 412

Walter, H.G., Hering, R., de Vegt, C., 1997, {\it A\&AS}, {\bf 122}, 529

\endreferences

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