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\input ibvs2.sty

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\IBVShead{5020}{25 January 2001}

\IBVStitletl{LO G\lowercase{em}: First Determination of the Orbital Period}{ and Light Curve}

\IBVSauth{Vandenbroere, Jacqueline; Beltraminelli, Nicola; Acerbi, Francesco}

\IBVSinsto{GEOS (Groupe Europ\'een d'Observation Stellaires),
23 Parc de Levesville, 28300 Bailleau l'Ev\"{e}que, France\\ \indent
j.vandenbroere@skynet.be, beltramine@dictagene.ch, acerbifr@tin.it} %email

\IBVSabs{Photoelectric and CCD measurements were carried out on LO Gem,}
\IBVSabs{an E star in the constellation of Gemini. We present}
\IBVSabs{the complete light curve and a model of the system.}

\SIMBADobj{LO Gem}
\IBVStyp{ EA }
\IBVSkey{photometry}

\begintext

LO Gem (GSC 1868:2176; 06\hr04\mm12\sec, +25\deg20\arcm, J2000) is
listed in the GCVS (Kholopov et al.\ 1985) as an E star varying from
11\fmm5 to 12\fmm0 (p).  The star was first reported to be an Algol
type binary system by Hoffmeister (1968). Ten times of minimum light
were determined from Sonneberg photographic plates taken between 1933
and 1967; however no period was derived (Gessner \& Meinunger 1973).
A first indication of the period of LO Gem was provided by the work
performed by several GEOS members, who collected 840 visual estimates,
thus allowing the determination of 16 times of minimum light
(Vandenbroere 1993). During five missions at Jungfraujoch Observatory
between 1992 and 1997, GEOS teams obtained 128 photoelectric
measurements in each of the $B$ and $V$ filters of the Geneva system,
covering the complete light curve of LO Gem.  These measurements were
carried out using the ``all sky'' method: the atmospheric absorption
coefficients were determined by measuring standard stars located at
different airmasses. Although no comparison stars were used, the good
quality of the photoelectric data (standard deviation about 0\fmm010
in both colours) could be evaluated from the stability of the atmospheric
absorption coefficients during the night.

Two series of 351 measurements in the $V$ and $I_C$ colours
(Johnson--Cousin system) were also performed with a CCD coupled to a
40-cm telescope in the observatory of one of the authors (N.B.,
Ghirone, Swiss Alps).  Differential magnitudes were obtained by using
four comparison stars, i.e.\ GSC 1868:2932, 1868:225, 1868:2279 and
1868:2280, whose positions are shown in the finder chart (Figure~1, S2
to S5, respectively).  The standard deviation of the measurements are
0\fmm014 for the first night (around phase 0.5) and 0\fmm017 for the
second one (around phase 0.0), as results from the analysis of the
comparison stars' data.
%\IBVSindexobj{GSC 1858-2932}
%\IBVSindexobj{GSC 1868-225}
%\IBVSindexobj{GSC 1868-2279}
%\IBVSindexobj{GSC 1868-2280}

To date, we have 3 photoelectric and 2 CCD minima at our disposal (they
were given a triple weight) together with 16 visual minima to determine
the  period elements of LO Gem.  We can refine this considering the
photographic minima reported by Hoffmeister (1968) as well as those
published by Gessner \& Meinunger (1973). The result of the linear
regression is the following:
$${\rm Min~I = HJD}~2427368.371(6) + 2\fday 2377825(18) \times E.$$ 
The uncertainty in the final digits are given in  brackets. The
photoelectric and CCD times of minima are reported in Table~1; the
first visual times are listed in Vandenbroere (1997) and the latest
one's will be submitted to the BBSAG Bulletin.

\IBVSfig{9.5cm}{5020-f1.ps}{Finder chart of LO Gem}

\begin{table}
\caption{Photoelectric and CCD minima of LO Gem}
\begin{center}
\begin{tabular}{llll}
\hline
HJD  &  Mode  &  $E$  &  $O-C$  \\
\hline
\\
2448978.6439 &   p.e.\ $V$  &   9657     &  0.0072  \\
2450342.5687 &   p.e.\ $V$  &  10266.5   &  0.0036  \\
2450462.2948 &   p.e.\ $V$  &  10320     &  0.0083  \\
2450811.3820 &   CCD  $V$  &  10476     &  0.0014  \\
2450812.5012 &   CCD  $V$  &  10476.5   &  0.0017  \\
\hline
\end{tabular}
\end{center}

\bigskip

\caption{Orbital Parameters. Asterisks mark ouput parameters}
\begin{center}
\begin{tabular}{ll}
\hline
Primary star  &   Secondary star \\
\hline
\multicolumn{2}{c}{
\begin{tabular}{r@{${}={}$}l}
mass ratio & 0.76* \\
inclination & 85\fdeg8* \\
wavelength & 550~nm \\ \end{tabular}} \\   
Fillout  = $-4.33$*            &    Fillout  = $-4.32$*             \\
Lagrangian $L_1 = 0.53$          &    Lagrangian $L_2 = 1.65$          \\
Mean radius  = 0.23          &    Mean radius  = 0.19          \\
temperature  = 7000~K*        &    temperature  = 6980~K*        \\
luminosity  = 0.60           &    luminosity  = 0.40           \\
gravity coefficient  =  0.32 &    gravity coefficient  =  0.32 \\
limb darkening  =  0.6       &    limb darkening  =  0.6       \\
reflection  =  0.5           &    reflection  =  0.5           \\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVS2fig{7.5cm}{5020-f2.ps}{Photoelectric $V$ measurements of LO Gem.  Different symbols indicate measurements taken during different nights}{5020-f3.ps}{$V$ band CCD light curve (points) with model (line) of the eclipsing system}

Two of the ten photographic times of minima had to be rejected from the
calculations because of their large $O-C$'s (+0.2638 d and +0.2377 d).
Taking together all the times of minima reported to date, the $O-C$'s
have not shown a significant period change since 1933.

Figure 2 shows the photoelectric $V$ light curve of LO Gem, constructed
using the ephemeris reported above. The  brightness of LO Gem outside
eclipse varies between $V=11.01$ and $V=11.06$. The standard deviation
of the $V$ measurements at maximum light is 0\fmm014. The amplitude of
the minima is 0\fmm60 at phase 0 and 0\fmm53 at phase 0.5; however, the
depth of the primary minimum changes significantly during the different
nights making it difficult to predict its correct amplitude (Figure~2).
Changes of the atmospheric absorption coefficients during short
time lapses can account for such variations; alternatively, we could
not rule out the possibility of the intrinsic variability of one of the
components, although the colour index of the system makes this
possibility unlikely.  The eclipse duration is 11.5\% of the orbital
period.

No significant changes in the $B-V$ colour index were detected during
the minima.  The $(B-V)_G$ colour index is $-0\fmm31 \pm 0.01$ at
maximum light, which corresponds to $(B-V)_J=0.52$. We used the
procedure  described by Meylan \& Hauck (1981), considering LO Gem as a
luminosity class V star. LO Gem belongs approximately to the spectral
class F8--G0.

Considering that the amplitudes of the  minima are almost identical
(Figure 2), we cannot exclude the possibility of the half value for
the  orbital period. In this case, the absence of a detectable
secondary minimum (corresponding to phases 0.25 and 0.75 in Figure 2)
prompted us to speculate that the obscure companion would be too faint
to be photoelectrically measurable. In order to obtain a more accurate
result on the construction of a model of the binary system, we also
considered the CCD measurements obtained during two subsequent minima.
With Binmaker 2.0 (Bradstreet 1993) the light curve was modelled using
both periods; the temperature of the larger star was set to  7000~K.
The data, considering the 2\fday23 period, were best fitted using an
inclination of 86\deg, a mass ratio of 0.76. The temperature of the
smaller star was adjusted to 6980~K. Figure 3 shows the fit of the CCD
measurements.  The main parameters of the system are reported in
Table~2.

Using the half period value and adding an artificial secondary minimum
of 0\fmm03 amplitude, we verified that the proximity effects  were
clearly visible on the modelled light curve, and these could never fit
the photoelectric and CCD data in a satisfactory way. Moreover, in the
case of a 0\fmm03 secondary minimum, a colour change should be clearly
detected due to the occultation of the cool companion whereas the
observations reveal no significant changes in the $B-V$ and $V-I_C$
colour indices.

We therefore conclude that LO Gem's orbital period is 2\fday23. The
shape of  minima indicate that the eclipses are nearly total. Being the 
amplitude of the minima 0\fmm6 we obtained an occultation of 85\%.

The binary system LO Gem is therefore an EA type with very similar
components of late F spectrum and an orbital period of 2\fday2377825.
Our study may not exclude that one or both star(s) is/are pulsating
variable(s). Future studies will address this possibility.

We acknowledge Paolo Bernasconi for his help in reducing the CCD
measurements, Roland Boninsegna and Joseph Remis for their help at
Jungfraujoch Observatory, Andrea Manna, Gilles Allenbach, Julie
Guignard and Carlo Barani for their contribution in this study.

\references

Bradstreet, D.H., 1993, {\it Binary Maker 2.0 User Manual}, Contact Software, Norristown, PA 19401-5505, USA

Kholopov, P.N. et al., 1985, GCVS, IV Edition


Gessner, H., Meinunger, I., 1973, {\it VSS}, {\bf  7}, 607, (H6)
%Modified: Tue Sep 20 10:40:32 MEST 2005

Hoffmeister, C., 1968, {\it Astron.\ Nach.}, {\bf 290}, 277

Meylan, G., Hauck, B., 1981, {\it A\&A Suppl.}, {\bf 46}, 281

Vandenbroere, J., 1993, {\it NC GEOS}, No.\ 710

Vandenbroere, J., 1997, {\it NC GEOS}, No.\ 837

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