\documentstyle[twoside, epsf]{article} 
 
\input ibvs2.sty 
 
\begin{document} 
\IBVShead{5413}{14 May 2003} 
 
\IBVStitle{V1154 T\lowercase{au}: a New Eclipsing Star within a Triple System}
  
\IBVSauth{Dallaporta, Sergio$^1$; Munari, Ulisse$^2$; Zwitter$^3$, Toma\v{z}}
 
\IBVSinst{Via Filzi 9, I-38034 Cembra (TN), Italy} 
\IBVSinst{Osservatorio Astronomico di Padova - INAF, Sede di Asiago, I-36032 Asiago (VI), Italy} 
\IBVSinst{University of Ljubljana, Department of Physics, Jadranska 19, 1000 Ljubljana, Slovenia}
 
\SIMBADobjAlias{V1154 Tau}{HD 32641}
\IBVStyp{ EA } 
\IBVSkey{photometry} 
\IBVSabs{V1154 Tau has been discovered by Hipparcos as a a low amplitude}
\IBVSabs{variable of unsolved nature. We have carried out extensive B,V}
\IBVSabs{photometry from Dec 2002 to February 2003 and discovered that}
\IBVSabs{V1154 Tau is an eclipsing system with an orbital period of }
\IBVSabs{1.7678805 days. The system is triple with the third component on an}
\IBVSabs{extremely wide orbit, 0.355 arcsec away, or 90 AU at the system }
\IBVSabs{distance, contributing 21\% of the total system light.  The orbit}
\IBVSabs{of the eclipsing binary is eccentric and the total color difference }
\IBVSabs{between primary and secondary eclipse is 0.02 mag in B-V, both being}
\IBVSabs{total with flat bottom.}

\begintext 

V1154~Tau (= HD 32641 = BD +22\fdeg818, spectral type B5) has been discovered as
a variable star by the Hipparcos satellite (HIP 23699, $V_T$=6\fmm72,
$B_T$=6\fmm80, $H_P$=6\fmm73) which did not however recognize its type of
variability. V1154~Tau was therefore logged as an ``unsolved" variable in the
{\em Hipparcos Catalogue} (ESA 1997), with an amplitude of $\Delta H_P$=
0.243 mag. V1154~Tau is located (J2000.0) at $\alpha$=05\hr 05\mm 37\fsec72 and
$\delta$=+23\deg 03\arcm 39\farcs8, corresponding to galactic coordinates $l$=179\fdeg90 and
$b$=--10\fdeg73. The parallax measured by Hipparcos is $\pi=4.02\pm1.21$,
for a distance of 250 pc. 

V1154~Tau long known as a close double, under the name Stt~97. Early
speckle observations by McAlister and DeGioia (1979) gave a separation
$\rho$=0.358$\pm$0.002~arcsec and a position angle $\theta$=154.1$\pm$0.3
deg for 1977. Hipparcos in 1991 obtained $\rho$=0.355$\pm$0.003 and
$\theta$=152.0$\pm$0.5, and Prieur et al. (2001) got $\rho$=0.355$\pm$0.003 and
$\theta$=150.4$\pm$0.8 with speckle observations in 1998. Such data do not show
evidence of orbital motion, nor different proper motion during the 21 elapsed
years, and therefore support a physical association of the pair (physical separation
$\sim$90 AU). Hipparcos has derived a 1.48 mag difference in $H_P$
magnitude for the two components. According to Fabricius and Makarov (2000),
the two components of the physical pair have:

\begin{itemize}
\item Component A: $H_P$=6.974$\pm$0.005, $B_T$=7.06$\pm$0.01, $V_T$=7.00$\pm$0.01
\item Component B: $H_P$=8.452$\pm$0.019, $B_T$=8.68$\pm$0.01, $V_T$=8.42$\pm$0.01
\end{itemize}

V1154~Tau lies in the vicinity of the NGC 1746/1750/1758 complex of possible
real open clusters. During a photometric investigation of such complex,
Strai\v{z}ys et al. (1992) obtained for V1154~Tau in the Vilnius photometric
system $V$=6.67, $U-P$=0.33, \hbox{$P-X$=0.43}, $X-Y$=0.28, $Y-Z$=0.18, $Z-V$=0.12,
$V-S$=0.25, from which they derived a B4~V classification, $M_V$=--0.8, $A_V$=0.99
and a distance of 280 pc, in good agreement with what later determined by
Hipparcos. Such a short distance rules out partnership with both NGC~1750
and NGC~1758 that should lie at 510 and 680 pc according to Strai\v{z}ys et
al. (1992; NGC 1746 does not seem to be a real cluster according to them).

We observed V1154~Tau in $B$ and $V$ (standard Johnson filters) from a
private observatory near Cembra (Trento), Italy, in a similar way to our
previous investigations of unsolved Hipparcos variables GV Dra and V432 Aur
(Dallaporta et al., 2000, 2002b) and the eclipsing binary GK~Dra with wrong
Hipparcos orbital period (Dallaporta et al., 2002a). The instrument was a 28
cm Schmidt-Cassegrain telescope equipped with an Optec SSP5 photometer. The
diaphragm had a size of 77 arcsec, and the exposure time was usually 
10~seconds. HD\,32811 (HIP\,23784, $V_J$=7\fmm146, $(B-V)_J=0.131$, spectrum
B9) was chosen as comparison star and HD\,32500 (HIP\,23606, $V_J$=7\fmm842,
$(B-V)_J=1\fmm120$, spectrum K0. As for HD\,32811, Jonhson's $B_J$, $V_J$
are derived from Tycho's values following Bessell (2000) transformations) as a check star. The comparison has been
measured by Hipparcos 102 times and found constant. We have measured it
against the check star five times in different nights and found
$V_J$=7\fmm843 (r.m.s. 0.009 mag) and $B_J$=8\fmm971 (r.m.s. 0.007~mag),
thus pretty well confirming the absence of variability.


\IBVSfig{13cm}{5413-f1.eps}{Our light and color curves of V1154~Tau for the $1\fday7678805$ period.}
%%\IBVSfigKey{5413-f1.eps}{V1154 Tau}{light curve}

All together, 511 measurements in $V$, and 518 in $B$ have been collected of
V1154~Tau between Dec. 2002 and Feb. 2003. All the observations were
corrected for atmospheric extinction and color corrections (via calibration
on Landolt's equatorial fields), and the instrumental differential
magnitudes were transformed into the standard Johnson UBV system. The
variable, comparison and check stars are very close on the sky so the
atmospheric corrections were rather small (36 arcmin distance for HD\,32811 and 38
arcmin for HD\,32500). The close similarity of the color between the
variable and comparison star and the fact that all observations have
been obtained for zenith distances $<60^\circ$ argue for a high internal
consistency of our photometry of V1154~Tau.

A Deeming-Fourier code has been applied to the set of data, resulting in the
detection of a strong periodicity at $1\fday76789$. Combining with Hipparcos
$H_P$ data it has been possible to refine the period to $1\fday7678805\pm 0.000002$,
with the ephemeris: 
$$ \begin{array}{r@{}r@{}l} 
{\rm Min.\ I = HJD\ }2452643.3792(\pm 0.0003) & {} + 1\fday7678805(\pm 0.000002) & {} \times E.
\end{array} $$ 

Our $B$ and $V$ photometric data folded to this ephemeris are presented in
Figure~1.  The system is clearly an eclipsing one, with both eclipses total
and very well marked. Phase plot of Hipparcos and Tycho data according to
the same ephemeris is presented in Figure~2. The eclipsing nature is well
evident in Hipparcos data too, and it is surprising that automatic data
treatment in preparation of the Hipparcos Catalogue has not solved the type
of variability affecting V1154~Tau.

\IBVSfig{13cm}{5413-f2.eps}{Hipparcos and Tycho data for V1154~Tau folded onto the $1\fday7678805$ period.}
%%\IBVSfigKey{5413-f2.eps}{V1154 Tau}{light curve}

Koen and Eyer (2002) have investigated with improved statistical methods a
large set of Hipparcos epoch photometry data, and found likely periodicities
for 2082 ``unsolved'' Hipparcos variables, including V1154~Tau for which
they determined a period of $1\fday8427$ (even if not assessing the nature
of the variability). However, when plotting our as well as Hipparcos data
according to Koen and Eyer (2002) period, the resulting light curve is not
what expected, indicating automatic detection of a wrong periodicity (cf. Figure~3).

The discovered eclipsing binary is the ``A'' speckle component listed above, and
therefore V1154~Tau turns out to be a triple system, with the third body
contributing a constant 21\% of the total system light in the $V$ band
light curve (our diaphragm obviously includes the triple system as a whole).
The phase of the secondary eclipse indicates an eccentric orbit. The $\Delta
(B-V)$=+0.01 at primary eclipse and $\Delta (B-V)$=--0.01 at secondary
suggests a small temperature difference between the two components, with
the hotter one being eclipsed at primary minimum. None of the components displays
intrinsic variability larger than 0.003 mag.

\IBVSfig{8cm}{5413-f3.eps}{Hipparcos $H_P$ and our $V$ band data for V1154~Tau folded onto the $1\fday8427$ period reported by Koen and Eyer (2002), evidently not the correct orbital one.}
%%\IBVSfigKey{5413-f3.eps}{V1154 Tau}{light curve}

Following these photometric results, V1154~Tau has been placed on the Asiago
eclipsing binary program (e.g. Munari et al. 2001). At the time of writing,
acquisition of radial velocities with the Asiago Echelle+CCD spectrograph is
progressing, while $B$,$V$ photometry is completed.  We present here only
the basic photometric evidences, with a full and detailed orbital solution
including radial velocity data and synthetic spectral analysis being
postponed to a devoted paper at the conclusion of the spectroscopic
campaign.

\vspace*{-0.25cm}

\references 

Bessell, M.S., 2000, {\it PASP}, {\bf 112}, 961

Dallaporta, S., Tomov, T., Zwitter, T., Munari U., 2000, {\it IBVS}, 4990  

Dallaporta, S., Tomov, T., Zwitter, T., Munari U., 2002a, {\it IBVS}, 5312  

Dallaporta, S., Tomov, T., Zwitter, T., Munari U., 2002b, {\it IBVS}, 5319

ESA, 1997, {\it The Hipparcos and Tycho Catalogues}, ESA SP-1200 

Fabricius, C., Makarov, V. V., 2000, {\it A\&A}, {\bf 356}, 141

Koen, C., Eyer, L., 2002, {\it MNRAS}, {\bf 331}, 45

McAlister, H. A., DeGioia, K. A., 1979, {\it ApJ}, {\bf 228}, 493

Munari, U., Tomov, T., Zwitter, T., Milone, E. F., Kallrath, J., et al., 2001, {\it A\&A}, {\bf 378}, 477  

Prieur, J.-L., Oblak, E., Lampens, P., et al., 2001, {\it A\&A}, {\bf 367}, 865

Strai\v{z}ys, V., Cernis, K., Meistas, E., 1992, {\it Baltic Astron.}, {\bf 1}, 125

\endreferences 

\end{document}
