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\input ibvs2.sty 
 
\begin{document} 
\IBVShead{5855}{20 October 2008} 
 
\IBVStitletl{Long term BVR$_{\rm C}$I$_{\rm C}$ photometry of carbon and symbiotic}{ stars in the Draco dwarf galaxy}
  
\IBVSauth{Munari,~U.$^1$; Siviero,~A.$^1$; Ochner, P.$^2$; Tomasoni,~S.$^2$; Moschini,~F.$^2$; Frigo, A.$^2$; Moretti,~S.$^2$; Tomaselli,~S.$^2$; Baldinelli,~L.$^2$; Maitan,~A.$^2$; Vagnozzi,~A.$^2$; Bacci,~S.$^2$}

\IBVSinst{INAF Osservatorio Astronomico di Padova, Sede di Asiago, I-36032 Asiago (VI), Italy} 
\IBVSinst{ANS Collaboration, c/o Osservatorio Astronomico, I-36032 Asiago (VI), Italy} 
 
\SIMBADobj{NAME Draco C1}
\SIMBADobj{NAME Draco C2}
\SIMBADobj{McClure C3} 
\NEDobj{UGC 10822}
\IBVStyp{ Carbon stars } 
\IBVSkey{photometry} 
\IBVSabs{We present the results of a BVRcIc CCD photometric monitoring of the}
\IBVSabs{symbiotic carbon star C1 and the carbon star C2 and C3 in the}
\IBVSabs{Draco dwarf galaxy. The observations extend over the time interval}
\IBVSabs{from 2006.30 to 2008.68. No obvious variability is affecting any of}
\IBVSabs{the three stars, with the except of C1 for which a variability with}
\IBVSabs{an amplitude Delta m <= 0.1 mag could be present.}

\begintext 
The Draco dwarf galaxy (Ddg) is a satellite of the Milky Way Galaxy,
characterized by low reddening ($E_{B-V}=0.03$), low metallicity
([Fe/H]\,$\approx-2$), high velocity dispersion
($\sigma_V=10.5$\,km\,sec$^{-1}$), a tidal radius of 40~arcmin
($\approx0.83$\,kpc), a total mass within the tidal radius of
$3.5\times 10^7 M_\odot$, and a distance modulus of $m-M=19.3$\,mag. The
overall light-to-mass ratio is $M/L=146\pm 42$ (in solar units), indicating
a strongly dark-matter dominated, bound stellar system (Odenkirchen et al.
2001).

The first three carbon stars (named C1, C2, C3) were discovered in Ddg by
Aaronson et al. (1982). Since then, other three were found (cf.
Kinemuchi et al. 2008, and references therein). One of the carbon stars, C1,
is a symbiotic binary (Belczynski et al. 2000). It displays a rich and high
ionization emission line spectrum (Aaronson et al. 1982, Munari and Buson
1994) and it is a super-soft X-ray source (Bickert et al. 1996).

An extensive search for variable stars in Ddg was carried out by Baade and
Swope (1961), during which they found 260 variables. They did not notice any
of the carbon stars as a variable star, in spite the mean brightness of the
variables they discovered and characterized (mainly RR~Lyr) was three whole
magnitudes fainter than the carbon stars themselves. Should any of the
carbon stars have varied by more than 0.2~mag, Baade and Swope's survey
would have detected them. The issue if any of the carbon stars in Ddg is
indeed variable, of very low amplitude, is still an open issue given the
contradicting results reported in the literature (see Kinemuchi et al. 2008
for a partial summary).

Over the last three years we have carried out a surveillance monitoring
of C1, looking for active phases of this highly interacting and energetic binary.

Our $BVR_{\rm C}I_{\rm C}$ CCD photometric surveillance extended from 2006.30 to 2008.68.
Together with C1, in the same field of view our CCD observations, we recorded
also C2 and C3. We observed with several telescopes, all located in
Italy: the 0.5\,m of Museo Civico di Rovereto (Trento), the 0.5\,m of
Osservatorio Astronomico S. Lucia di Stroncone (Terni), the 0.4\,m of
Associazione Ravennate Astrofili Rheyta in Bastia (Ravenna), the 0.4\,m of
Osservatorio Astronomico Pizzinato (Bologna), and a 0.3\,m located in Folgaria
(Trento). All instruments were equipped with either Schuler, Optec, Custom
Scientific or Omega standard $BVR_{\rm C}I_{\rm C}$ filters. All observations were reduced
and corrected for color equations using the same photometric comparison
sequence calibrated around C1 by Henden and Munari (2000). A total 121 $VR_{\rm C}I_{\rm C}$
runs were collected in separate nights, 33 of which included also
observations in the $B$ band.

\begin{table}
\caption{Median values (and errors) of our $BVR_{\rm C}I_{\rm C}$ photometry of Draco C1, C2
and C3 covering the period from 2006.30 to 2008.68. Median values (and
errors) from Skopal et al. (2007, coded $b$) and mean values (and errors)
from Kinemuchi et al. (2008, coded $a$) are given for comparison.}
%\center
\begin{center}
\begin{tabular}{lllllll}
\multicolumn{6}{c}{}\\
\hline
\multicolumn{6}{c}{}\\
\multicolumn{1}{c}{} & 
\multicolumn{1}{c}{$<V>$} & 
\multicolumn{1}{c}{$<I_{\rm C}>$} & 
\multicolumn{1}{c}{$<B-V>$} & 
\multicolumn{1}{c}{$<V-I_{\rm C}>$} & 
\multicolumn{1}{c}{$<R_{\rm C}-I_{\rm C}>$}\\
\multicolumn{6}{c}{}\\
C1 & 17.17 ~~0.08 & 15.68 ~~0.05 & 1.49 ~~0.14 & 1.49 ~~0.06 & 0.63 ~~0.06 &     \\
C2 & 17.36 ~~0.06 & 16.03 ~~0.05 & 1.59 ~~0.25 & 1.31 ~~0.07 & 0.63 ~~0.05 &     \\
C3 & 17.53 ~~0.09 & 16.23 ~~0.08 & 1.47 ~~0.29 & 1.31 ~~0.10 & 0.59 ~~0.06 &     \\
\multicolumn{6}{c}{}\\
C1 & 17.15 ~~0.08 & 15.66 ~~0.04 &             & 1.49 ~~0.09 &             & $a$ \\
C2 & 17.30 ~~0.05 & 15.99 ~~0.04 &             & 1.31 ~~0.06 &             & $a$ \\
\multicolumn{6}{c}{}\\
C1 & 17.19 ~~0.05 & 15.82 ~~0.07 & 1.45 ~~0.03 & 1.41 ~~0.05 & 0.65 ~~0.02 & $b$ \\
\multicolumn{6}{c}{}\\
\hline
\end{tabular}
\end{center}
\end{table}

Even if no outburst or bright phase of C1 has been recorded, the collected
data allow to put constraints on the variability of C1, C2 and C3. Table~1
lists the median values of our observations, and for comparison the median
values obtained by Skopal et al. (2007), who monitored C1 from 2003.9 to
2007.2, and by Kinemuchi et al. (2008), who monitored C1 and C2 from 1993 to
1996. The values in Table~1 agree well within the errors, indicating the
absence of any long term trend affecting C1 and C2 over the last 15 years.

The program stars were faint, our telescopes of limited diameter and the
observations were carried out in surveillance, short-exposure mode.
Therefore, the error affecting the single photometric point is significant.
To obtain meaningful light-curves is necessary to bin the data. A bi-monthly
binning proved to be the most convenient in term of noise suppression and
preservation of light-curve details. Such a bi-monthly binning provides the
following results for the three program stars.

C3 is not variable, at least not with an amplitude larger than 0.05\,mag.

C2 is a border-line case. The amplitude of any actual variability
should not exceed 0.1\,mag. Shetrone et al. (2001) listed C2 as a
definite photometric variable, but they did not provide supporting
details. An amplitude around 0.2\,mag is listed for C2 by Kinemuchi et
al. (2008), again with not details.

C1 is more confidently a true variable, as illustrated by Figure~1 which
could also support feeble hints of an anti-correlation of brightness and
color (C1 redder when fainter, bluer when brighter), as observed in
pulsating stars. The amplitude is $\approx 0.2$\,mag, with a possible
periodicity of the order of one year. Amplitude and period would be
appropriate for either a reflection effect or an ellipsoidal distortion of
the carbon star. It is worth noticing that Shetrone et al. (2001) asserted
C1 to be not variable, while it is definitively a variable according to Kinemuchi et
al. (2008).

The reflection effect is quite common in symbiotic binaries and it is caused
by the very hot and luminous white dwarf illuminating the facing side of the
cool giant companion.  In this case the period of the reflection effect is 
also the orbital period. The ellipsoidal distortion of the mass donor
cool giant is also frequently observed in symbiotic binaries, where the high
mass transfer rate necessary to sustain the stable H-burning conditions on
the white dwarf, requires Roche lobe filling conditions. In this case, the
orbital period is twice longer than the period of photometric variability.
Given its super-soft X-ray nature, both a reflection effect and the Roche
lobe filling are highly probable to occur in C1, and the light-curve in
Figure~1 could result from the combination of the two. The high-resolution
spectroscopic observations by Munari (1991) proved C1 to be variable in both
radial velocity and emission line profiles. Clearly, C1 is worth further
observations, which we plan to carry out.

\IBVSfig{9cm}{5855-f1.ps}{$V$, $V-I_{\rm C}$ light-curve of carbon symbiotic binary C1
from our observations, binned into bi-monthly means. The size of the dots
is proportional to the number of observations in that bin. The error bars
are the errors of the mean.}
 

\references 

 Aaronson, M., Liebert, J., Stocke, J., 1982, ApJ, 254, 507

 Baade, W., Swope, H. H., 1961, AJ, 66, 300

 Belczynski, K., Mikolajewska, J., Munari, U. et al.,  2000, A\&AS, 146, 407

 Bickert, K. F., Greiner, J., Stencel, R. E., 1996, Lecture Notes in Physics, 
472, 225, in: Supersoft X-Ray Sources, J. Greiner ed., Springer-Verlag 

 Henden, A., Munari, U.,  2000, A\&AS, 143, 343

 Kinemuchi, K., Harris, H. C., Smith, H. A. et al., 2008, AJ, 136, 1921

 Munari, U., 1991, A\&A, 251, 103

 Munari, U., Buson, L. M., 1994, A\&A, 287, 87

 Odenkirchen, M., Grebel, E. K., Harbeck, D. et al., 2001, AJ, 122, 2538

 Shetrone, M. D., Cote, P., Stetson, P. B., 2001, PASP, 113, 1122

 Skopal, A., Vanko, M., Pribulla, T. et al., 2007, AN, 328, 909

\endreferences 

\end{document}
