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\begin{document}

\IBVShead{5867}{15 December 2008}

\IBVStitletl{A multicolor photometric study of CN Orionis}
%\vskip -4mm

\IBVSauth{Spogli, Corrado$^{1,2}$; Fiorucci, Massimo$^{1,3}$; Capezzali, Daniele$^{1,2}$;\\ Nucciarelli, Giuliano$^1$; Mancinelli, Vannio$^2$; Brunozzi, Paolo$^2$; Fagotti, Paolo$^2$}

\IBVSinst{Physics Dept. and Astronomical Observatory, University of Perugia, Via A. Pascoli, 06123 Perugia, Italy}
\IBVSinst{Porziano Astronomical Observatory, Via Santa Chiara 2, 06081 Assisi, PG, Italy}
\IBVSinst{INAF Osservatorio Astronomico di Padova, Via dell'Osservatorio 8, 36012 Asiago, VI, Italy}
\vskip -4mm

\SIMBADobj{CN Ori}
\IBVSkey{photometry}

\IBVSabs{In this paper we report BVRI observations and photometric analysis}
\IBVSabs{of CN Ori, one of the brightest cataclysmic systems above the}
\IBVSabs{period gap. A comparison sequence and finding chart is included.}

\begintext
CN Ori is one of the brightest and most active dwarf novae (DNe), with a very short 
recurrence time between two consecutive outbursts ($\simeq$ 18 days), and one of
the few DNe with an orbital period above the 3-hour upper boundary of the period gap 
($P = 0\fday 163199$). The maximum reported brightness is $V=11.9$. The minimum is 
extremely variable and ranges between $V=14.2$ and $V=16.3$, with a quasi--sinusoidal 
component of variability due to a partial eclipse (Mumford 1967), and other components 
superimposed that probably are the signature of an accretion disk out of the steady state
(Mantel et al. 1988).
Many photometric observations of CN~Ori are reported in the literature, with time 
scales ranging from a few seconds to many days (see e.g., Petit 1960, Shoembs 1982). 
However, these data have been generally obtained in a single photometric band, and 
only sporadic multi--band observations are available (for example, Echevarria 1984). 

We have observed this dwarf nova at the Perugia Astronomical Observatory since 2002. 
The instruments used and the photometric techniques applied was already described in 
Spogli et al. (1998). Other observations have been collected at the Porziano Astronomical 
Observatory, Mt. Subasio, Assisi (Italy), with a 0.35\,m Schmidt--Cassegrain telescope 
equipped with an HiSIS 23 CCD Camera (Kodak Kaf 401E of 762$\times$512 pixel) and standard 
$BVR_CI_C$ Johnson--Cousins broad band filters. There is no evaluable difference between 
the reduced data obtained with the two telescopes.

\IBVSfig{6.5cm}{5867-f1.ps}{Finding chart of CN~Ori, with the selected sequence.}
\IBVSfigKey{5867-f1.ps}{CN Ori}{finding chart}

The photometric data have been obtained in differential photometry using the calibration 
stars reported by Bailey \& Howarth (1979) and Misselt (1996). With the principal aim to 
give the $I_C$ magnitudes, we selected and re--calibrated a new sequence by observing,
on three photometric nights, several standard stars (Landolt 1992) having $B-V$ from 
$-0.2$ to $1.5$ mag. Table~1 reports the weighted averages and the standard deviations for the 
selected comparison stars. Fig.~1 shows the finding chart. 
The $BVR_C$ magnitudes are in general agreement with the results of Misselt (1996), while 
they are in average fainter by 0.07 mag compared with the $B$ and $V$ photoelectric estimates 
of Bailey \& Howarth (1979), with an r.m.s. scatter of 0.08 mag around the offset. The
$I_C$ values are original results.

Table 2 gives the $BVR_CI_C$ magnitudes of CN~Ori. An important sample of our data has been
obtained during the partial eclipse of the binary system, considering the orbital ephemeris
reported by Barrera \& Vogt (1989). Neglecting eclipse points, our data confirm the very short
recurrence time between two consecutive outbursts (Fourier periodogram gives 
$P \simeq 18.76$ days), with a symmetric shape of the outburst profile characterized by a 
relatively fast rise and decline. Many variability patterns are superimposed on the outburst 
cycle, but our data--sampling is not applicable for an accurate investigation. Fig.~2 shows that
the outburst amplitude changes at different filters. Fig.~3 shows the color-index versus 
magnitude diagram for CN~Ori.

\begin{table}[bh!!]
\begin{center}
{\normalsize Table 1: $BVR_CI_C$ magnitudes for the selected comparison stars}\\
\smallskip
\small
\begin{tabular}{lcccccc}
\hline
name(s) & RA        & DEC      & $B$ & $V$ & $R_C$ & $I_C$ \\
\hline
C1, M1$^{(1)}$, L$^{(2)}$  & {\small 5:52:04.0} & {\small -5:25:40} & 15.95$\pm$0.06 & 14.65$\pm$0.05 & 13.88$\pm$0.03 & 13.15$\pm$0.03\\
C2, M$^{(2)}$              & {\small 5:52:00.0} & {\small -5:26:42} & 16.08$\pm$0.07 & 15.10$\pm$0.04 & 14.52$\pm$0.04 & 14.01$\pm$0.05\\
C3                         & {\small 5:51:57.0} & {\small -5:27:40} & 14.19$\pm$0.08 & 12.94$\pm$0.04 & 12.26$\pm$0.02 & 11.58$\pm$0.03\\
C4, M3$^{(1)}$             & {\small 5:52:10.7} & {\small -5:26:56} & 12.93$\pm$0.03 & 12.48$\pm$0.03 & 12.19$\pm$0.03 & 11.97$\pm$0.03\\
C5                         & {\small 5:52:10.5} & {\small -5:30:03} & 10.32$\pm$0.05 &  9.81$\pm$0.03 &  9.50$\pm$0.05 &  9.24$\pm$0.03\\
C6, C$^{(2)}$              & {\small 5:52:38.8} & {\small -5:31:09} & 13.15$\pm$0.10 & 11.93$\pm$0.03 & 11.12$\pm$0.04 & 10.34$\pm$0.03\\
C7, B$^{(2)}$              & {\small 5:52:31.3} & {\small -5:28:32} & 11.78$\pm$0.07 & 11.34$\pm$0.04 & 10.98$\pm$0.03 & 10.76$\pm$0.04\\
C8, G$^{(2)}$              & {\small 5:52:21.4} & {\small -5:26:20} & 14.90$\pm$0.05 & 13.68$\pm$0.04 & 12.92$\pm$0.02 & 12.19$\pm$0.03\\
C9, M5$^{(1)}$, H$^{(2)}$  & {\small 5:52:17.6} & {\small -5:26:18} & 15.36$\pm$0.05 & 13.93$\pm$0.03 & 13.14$\pm$0.03 & 12.38$\pm$0.03\\
C10, M4$^{(1)}$, K$^{(2)}$ & {\small 5:52:12.1} & {\small -5:25:21} & 15.45$\pm$0.05 & 14.21$\pm$0.04 & 13.48$\pm$0.03 & 12.74$\pm$0.03\\
C11                        & {\small 5:52:18.5} & {\small -5:24:10} & 14.88$\pm$0.09 & 13.92$\pm$0.04 & 13.33$\pm$0.04 & 12.85$\pm$0.04\\
C12, A$^{(2)}$             & {\small 5:51:58.1} & {\small -5:21:08} & 11.76$\pm$0.03 & 10.61$\pm$0.02 &  9.94$\pm$0.03 &  9.29$\pm$0.03\\
\hline
\end{tabular}
\end{center}
(1) Misselt (1986)\\
(2) Bailey \& Howarth (1979)
\end{table}

\AladinFindingChart{5867-t1}
\IBVSecomment{The stars in Table 1 viewed with CDS Aladin}


\IBVSfig{8cm}{5867-f2.ps}{The phase diagram of CN~Ori shows a relatively
stable outburst-cycle, with a quasi-sinusoidal trend (dashed line), and the outburst 
amplitude that changes at different wavelengths. Empty circles represent 
the data obtained during the partial eclipse of the binary system.}
\IBVSfigKey{5867-f2.ps}{CN Ori}{phased light curve}



\IBVSfig{8.5cm}{5867-f3.ps}{During the outburst the emission is dominated by the accretion disk and the
color index is bluer; the emission of the secondary is progressively more important during 
quiescence and the color index is redder, with a complex behaviour like a curvature (dotted line).}
\IBVSfigKey{5867-f3.ps}{CN Ori}{color-magnitude diagram}



\begin{table}[th]
\begin{center}
{\normalsize Table 2: $BVR_CI_C$ data of CN Ori}\\
\smallskip
\small
\begin{tabular}{lccccc}
\hline
UT DATE    & JD (245+)&      $B$       &      $V$       &    $R_C$       &     $I_C$     \\
\hline
11/02/2002 & 2317.327 & 12.78$\pm$0.06 & 12.77$\pm$0.05 & 12.68$\pm$0.04 & 12.62$\pm$0.04\\
22/02/2002 & 2328.339 & 15.60$\pm$0.07 & 15.31$\pm$0.05 & 14.90$\pm$0.05 & 14.56$\pm$0.05\\
24/02/2002 & 2330.332 & 15.15$\pm$0.07 & 14.85$\pm$0.05 & 14.42$\pm$0.05 & 14.16$\pm$0.04\\
04/03/2002 & 2338.326 & 13.03$\pm$0.07 & 12.95$\pm$0.05 & 12.85$\pm$0.04 & 12.77$\pm$0.04\\
10/03/2002 & 2344.361 &                & 15.94$\pm$0.08 & 15.44$\pm$0.05 & 14.79$\pm$0.05\\
19/12/2002 & 2628.458 &                &                & 14.74$\pm$0.04 &               \\
10/01/2003 & 2650.443 &                &                & 14.48$\pm$0.05 &               \\
10/01/2003 & 2650.485 &                &                & 14.35$\pm$0.04 &               \\
18/01/2003 & 2658.434 &                &                & 14.21$\pm$0.05 &               \\
12/03/2003 & 2711.385 &                &                & 12.70$\pm$0.03 &               \\
05/12/2003 & 2979.499 & 13.53$\pm$0.06 & 13.45$\pm$0.04 & 13.39$\pm$0.04 & 13.27$\pm$0.04\\
20/12/2003 & 2994.488 &                & 12.81$\pm$0.05 & 12.71$\pm$0.05 & 12.63$\pm$0.05\\
26/12/2003 & 3000.428 & 15.61$\pm$0.08 & 15.11$\pm$0.03 & 14.72$\pm$0.04 & 14.31$\pm$0.03\\
23/01/2004 & 3028.444 & 12.88$\pm$0.07 & 12.74$\pm$0.04 & 12.62$\pm$0.04 & 12.52$\pm$0.03\\
23/01/2004 & 3028.478 & 12.99$\pm$0.07 & 12.80$\pm$0.05 & 12.69$\pm$0.04 & 12.60$\pm$0.04\\
24/01/2004 & 3029.298 & 13.05$\pm$0.06 & 12.85$\pm$0.04 & 12.73$\pm$0.04 & 12.66$\pm$0.04\\
24/01/2004 & 3029.317 & 12.95$\pm$0.06 & 12.79$\pm$0.04 & 12.65$\pm$0.04 & 12.60$\pm$0.04\\
24/01/2004 & 3029.335 & 12.88$\pm$0.06 & 12.77$\pm$0.04 & 12.68$\pm$0.04 & 12.62$\pm$0.04\\
24/01/2004 & 3029.358 & 12.85$\pm$0.06 & 12.75$\pm$0.04 & 12.66$\pm$0.04 & 12.57$\pm$0.04\\
30/01/2004 & 3035.431 & 15.28$\pm$0.08 & 14.77$\pm$0.05 & 14.50$\pm$0.04 & 14.17$\pm$0.04\\
30/01/2004 & 3035.442 & 15.09$\pm$0.06 & 14.63$\pm$0.04 & 14.44$\pm$0.04 & 14.16$\pm$0.03\\
12/02/2004 & 3048.362 & 12.93$\pm$0.06 & 12.89$\pm$0.04 & 12.82$\pm$0.04 & 12.79$\pm$0.03\\
13/02/2004 & 3049.384 & 13.35$\pm$0.04 & 13.34$\pm$0.03 & 13.20$\pm$0.04 & 13.10$\pm$0.03\\
16/02/2004 & 3052.365 & 15.21$\pm$0.07 & 14.82$\pm$0.05 & 14.50$\pm$0.05 & 14.11$\pm$0.04\\
17/02/2004 & 3053.426 & 15.82$\pm$0.08 & 15.24$\pm$0.05 & 14.85$\pm$0.05 & 14.49$\pm$0.05\\
02/03/2004 & 3067.313 & 13.10$\pm$0.06 & 12.99$\pm$0.04 & 12.89$\pm$0.03 & 12.82$\pm$0.03\\
05/03/2004 & 3070.298 &                & 14.64$\pm$0.05 & 14.39$\pm$0.04 & 14.03$\pm$0.05\\
27/11/2005 & 3702.525 & 12.46$\pm$0.06 & 12.43$\pm$0.04 & 12.41$\pm$0.04 & 12.39$\pm$0.04\\
28/11/2005 & 3703.457 & 12.58$\pm$0.05 & 12.50$\pm$0.05 & 12.49$\pm$0.05 & 12.48$\pm$0.04\\
07/12/2005 & 3712.458 &                & 15.59$\pm$0.07 & 15.19$\pm$0.05 & 14.76$\pm$0.05\\
10/01/2006 & 3746.375 & 13.06$\pm$0.06 & 12.93$\pm$0.05 & 12.86$\pm$0.05 & 12.75$\pm$0.05\\
11/01/2006 & 3747.376 & 13.17$\pm$0.06 & 13.06$\pm$0.05 & 13.04$\pm$0.05 & 12.96$\pm$0.05\\
12/01/2006 & 3748.332 & 13.56$\pm$0.06 & 13.46$\pm$0.05 & 13.40$\pm$0.05 & 13.32$\pm$0.05\\
13/01/2006 & 3749.429 & 14.52$\pm$0.06 & 14.39$\pm$0.06 & 14.25$\pm$0.05 & 14.05$\pm$0.05\\
20/01/2006 & 3756.357 &                & 15.02$\pm$0.05 & 14.65$\pm$0.05 & 14.25$\pm$0.05\\
22/01/2006 & 3758.374 & 13.38$\pm$0.07 & 13.23$\pm$0.05 & 13.18$\pm$0.05 & 13.08$\pm$0.05\\
31/01/2006 & 3767.361 & 14.16$\pm$0.07 & 14.02$\pm$0.05 & 13.92$\pm$0.05 & 13.75$\pm$0.05\\
01/02/2006 & 3768.343 &                & 14.68$\pm$0.06 & 14.38$\pm$0.05 & 14.12$\pm$0.05\\
02/02/2006 & 3769.388 &                & 15.52$\pm$0.06 & 15.10$\pm$0.06 & 14.57$\pm$0.05\\
\hline
\end{tabular}
\end{center}
\end{table}



%\pagebreak





%\begin{samepage}


During the rise to the outburst, and the subsequent decline, the emission is progressively 
dominated by the accretion disk and the color index follows the typical path well represented 
by a line. During quiescence the emission is generally dominated by the secondary, but our data
show a large variation in the color--magnitude diagram, with a change in slope and curvature, 
and the color index is far from the expected color of an M4-5 star (Ritter \& Kolb 1998).
So we can conclude that the accretion disk remains relatively bright during the quiescence of 
CN~Ori, and the large color variations are increased by the partial eclipse of the binary 
system.





\references

Bailey, J., Howarth, F. D. 1979, J. British Astron. Assoc., 89, 265

Barrera, L. H., Vogt, N. 1989, A\&A, 220, 99

Echevarria, J. 1984, Rev. Mexicana Astron. Astrof., 9, 99

Landolt, A. U. 1992, AJ, 104, 340

Mantel, K. H., Marschhausser, R. K., Shoembs, R., et al. 1988, A\&A, 193, 101

Misselt, K. A. 1996, PASP, 108, 146

Mumford, G. S. 1967, PASP, 79, 283

Petit, M. 1960, JO, 43, 24

Ritter, H., Kolb, U.  1998, A\&AS, 129, 83

Schoembs, R. 1982, A\&A, 115, 190

Spogli, C., Fiorucci, M., Tosti, G. 1998, A\&AS, 130, 485

\endreferences


\IBVSedata{5867-t2.txt}
\IBVSdataKey{5867-t2.txt}{CN Ori}{photometry}


\end{document}
