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\begin{document}

\IBVShead{5749}{22 January 2007}

\IBVStitle{BVRI Photometry of VW V\lowercase{ul} and New Comparison Stars}

\IBVSauth{Capezzali, D.$^{1,2}$; Spogli, C.$^{1,2}$; Fiorucci, M.$^1$; Ciprini, S.$^1$; Nucciarelli, G.$^1$;\\ Mancinelli, V.$^2$; Brunozzi, P.$^2$; Fagotti, P.$^2$; Brandoni, L.$^2$; Rocchi, G.$^2$}

\IBVSinst{Physics Department and Astronomical Observatory,
University of Perugia, Perugia, Italy}

\IBVSinst{Porziano Astronomical Observatory, Via Santa Chiara 2
Assisi, Italy}

\SIMBADobj{VW Vul}
\IBVStyp{ UGZ }
\IBVSkey{photometry}

\IBVSabs{In this paper we report BVRI observations of the dwarf}
\IBVSabs{nova VW Vulpeculae. Moreover, new comparison stars are calibrated} 
\IBVSabs{in the VRI bands and included in the text with the finding chart.}

\begintext

The dwarf nova VW Vulpeculae is classified as Z Cam (UGZ) in the GCVS (Kholopov
et al., 1985--1990), with $B$ magnitudes ranging from 13.1 to 16.27. Shafter
(1985) published a spectroscopic study and reported a period of 0.0731 day.
However, Thorstensen et al.\ (1998) computed an orbital period of 0.1687 day
from the measurement of H$\alpha$ radial velocities in quiescence. Only a few
photometric data are available for this source. Wenzel (1985) found a
19${}\pm{}$5 days cycle length on 40 years of archival plates. Bruch \& Engel
(1994) report $B-V=0.12$ during the outburst, and $B-V=0.35$ in quiescence. More
recently, Kato (1999) gives the light curve of VW Vul during the 1995
standstill.

With the aim to increase the multi-band photometric database of VW Vul, we
observed this source at the Porziano Astronomical Observatory during the summers
of 2004 and 2005. The photometric system consists of an 0.35-m
Schmidt--Cassegrain telescope, equipped with an HiSIS 23 CCD camera (Kodak Kaf
401E of $762\times512$ pixels) and $B$, $V$, $R_c$, $I_c$ Johnson--Cousins
broad-band filters. The exposure time was 120--300 s depending on the brightness
of the object. The frames were first corrected for bias and flat-field, and then
processed by a PC-based aperture photometry package developed by one of the
authors using DAOPHOT routines (Stetson, 1987).

Few other observations were obtained with the AIT at the Perugia University
Observatory (see Spogli et al., 1998 for a description of instruments and
data-reduction). There is no evaluable difference between the reduced data
obtained with the two different telescopes.

\begin{table}[p]
\begin{center} {\normalsize Table 1: New comparison stars of VW
Vul}
\smallskip
\begin{tabular}{lcccccc} 
\hline
 No.  &\multicolumn{2}{c}{J2000.0 coord.}& $V$ & $R_c$ & $I_c$ & Obs.\\
      & $\alpha$ & $\delta$ & [mag] & [mag] & [mag] & nights\\ \hline
 C1 & 20\hr 57\mm 32\fsec82 &+25\deg 30\arcm 20\farcs3 & 14.26${}\pm{}$0.02 &
13.86${}\pm{}$0.02 & 13.44${}\pm{}$0.03 & 17\\
 C2 & 20\hr 57\mm 28\fsec48 &+25\deg 33\arcm 27\farcs8 & 15.17${}\pm{}$0.05 &
14.59${}\pm{}$0.02 & 14.03${}\pm{}$0.04 & 16\\
 C3 & 20\hr 57\mm 18\fsec30 &+25\deg 29\arcm 49\farcs0 & 15.39${}\pm{}$0.03 &
14.92${}\pm{}$0.01 & 14.44${}\pm{}$0.03 & 10\\
 C4 & 20\hr 57\mm 20\fsec95 &+25\deg 28\arcm 52\farcs5 & 13.53${}\pm{}$0.03 &
12.49${}\pm{}$0.02 & 11.58${}\pm{}$0.03 & 14\\
 C5 & 20\hr 57\mm 14\fsec35 &+25\deg 29\arcm 36\farcs3 & 16.02${}\pm{}$0.05 &
15.64${}\pm{}$0.02 & 15.27${}\pm{}$0.03 & 5\\
 C6 & 20\hr 57\mm 21\fsec35 &+25\deg 26\arcm 34\farcs6 & 15.41${}\pm{}$0.04 &
14.90${}\pm{}$0.03 & 14.41${}\pm{}$0.03 & 9\\
 \hline
\end{tabular}
\end{center}

\bigskip

\begin{center}
{\normalsize Table 2: Photometric data of VW Vulpeculae}
\smallskip
\begin{tabular}{cccccc}
\hline
 UT Date   &   JD       & $B$        & $V$         & $R_c$       & $I_c$ \\
           &(2453000+)  &            &             &             &       \\
\hline
25/06/2004 &181.526&15.19${}\pm{}$0.08&14.77${}\pm{}$0.02&14.49${}\pm{}$0.02&14.33${}\pm{}$0.03\\
26/06/2004 &182.556&15.08${}\pm{}$0.08&14.72${}\pm{}$0.04&14.48${}\pm{}$0.03&14.24${}\pm{}$0.03\\
27/06/2004 &184.421&14.79${}\pm{}$0.08&14.50${}\pm{}$0.02&14.28${}\pm{}$0.02&14.10${}\pm{}$0.03\\
28/06/2004 &185.413&14.64${}\pm{}$0.09&14.28${}\pm{}$0.03&14.11${}\pm{}$0.02&13.90${}\pm{}$0.03\\
01/07/2004 &188.485&14.34${}\pm{}$0.07&13.99${}\pm{}$0.03&13.85${}\pm{}$0.03&13.59${}\pm{}$0.04\\
02/07/2004 &189.457&14.56${}\pm{}$0.07&14.25${}\pm{}$0.05&14.05${}\pm{}$0.03&13.81${}\pm{}$0.04\\
05/07/2004 &192.411&15.11${}\pm{}$0.08&14.76${}\pm{}$0.03&14.48${}\pm{}$0.02&14.27${}\pm{}$0.04\\
06/07/2004 &193.417&              &14.84${}\pm{}$0.03&14.54${}\pm{}$0.04&14.24${}\pm{}$0.05\\
07/07/2004 &194.437&              &14.97${}\pm{}$0.03&14.70${}\pm{}$0.04&14.37${}\pm{}$0.04\\
09/07/2004 &196.475&              &15.19${}\pm{}$0.04&14.85${}\pm{}$0.02&14.50${}\pm{}$0.03\\
10/07/2004 &197.473&15.38${}\pm{}$0.08&14.96${}\pm{}$0.02&14.64${}\pm{}$0.02&14.38${}\pm{}$0.03\\
13/07/2004 &200.437&              &15.30${}\pm{}$0.10&14.98${}\pm{}$0.04&14.55${}\pm{}$0.04\\
15/07/2004 &202.406&15.28${}\pm{}$0.10&14.79${}\pm{}$0.02&14.52${}\pm{}$0.02&14.18${}\pm{}$0.02\\
17/07/2004 &204.471&13.93${}\pm{}$0.07&13.71${}\pm{}$0.02&13.57${}\pm{}$0.02&13.44${}\pm{}$0.02\\
21/07/2004 &208.426&14.88${}\pm{}$0.08&14.53${}\pm{}$0.02&14.30${}\pm{}$0.02&14.08${}\pm{}$0.02\\
23/07/2004 &210.443&14.68${}\pm{}$0.08&14.47${}\pm{}$0.05&14.34${}\pm{}$0.05&13.99${}\pm{}$0.04\\
\hline
14/08/2005 &596.525&              &15.77${}\pm{}$0.03&15.39${}\pm{}$0.03&15.02${}\pm{}$0.04\\
15/08/2005 &597.534&15.77${}\pm{}$0.10&15.40${}\pm{}$0.04&15.01${}\pm{}$0.04&14.61${}\pm{}$0.04\\
16/08/2005 &599.441&15.63${}\pm{}$0.10&15.15${}\pm{}$0.02&14.86${}\pm{}$0.02&14.64${}\pm{}$0.03\\
09/09/2005 &623.420&15.84${}\pm{}$0.05&15.32${}\pm{}$0.02&15.03${}\pm{}$0.03&14.66${}\pm{}$0.03\\
10/09/2005 &624.428&15.65${}\pm{}$0.05&15.22${}\pm{}$0.03&15.06${}\pm{}$0.02&14.71${}\pm{}$0.04\\
23/09/2005 &637.415&              &15.36${}\pm{}$0.05&15.06${}\pm{}$0.05&14.75${}\pm{}$0.04\\
26/09/2005 &640.398&              &14.61${}\pm{}$0.05&              &14.24${}\pm{}$0.04\\
29/10/2005 &673.379&              &15.17${}\pm{}$0.03&14.92${}\pm{}$0.02&14.57${}\pm{}$0.05\\
19/11/2005 &694.261&14.68${}\pm{}$0.07&14.34${}\pm{}$0.02&14.16${}\pm{}$0.03&13.97${}\pm{}$0.03\\
10/11/2005 &695.230&14.95${}\pm{}$0.05&14.57${}\pm{}$0.02&14.30${}\pm{}$0.02&14.10${}\pm{}$0.04\\
\hline
\end{tabular}
\end{center}
\end{table}

\IBVSfig{10.2cm}{5749-f1.ps}{New comparison stars to be
added to the Misselt (1996) sequence. North is up and East to the left.
The frame is $11\arcm \times 8\arcm$}
\IBVSfigKey{5749-f1.ps}{VW Vul}{finding chart}

\IBVSfig{9cm}{5749-f2.ps}{$V$ light curve of VW Vul in
summer 2004. Filled circles are our data, while small crosses are
visual estimates available from AFOEV ({\tt cdsweb.u-strsbg.fr/afoev}). 
The variable was observed during the rise to a low-amplitude outburst, 
the successive decline and the following fast burst. Error bars show 
the standard deviations}
\IBVSfigKey{5749-f2.ps}{VW Vul}{light curve}

\vspace{-22pt}

All the data of VW Vul here reported were obtained in differential photometry
using the calibration stars given by Misselt (1996) with the numbers M2, M3, M6,
M7. Moreover, we calibrated these comparison stars with the $I_c$ filter by
observing, on different photometric nights, several standard stars (Landolt,
1992) having $B-V$ from $-0.2$ to 1.4, over a wide range of airmass. The
weighted averages are:  $I_c({\rm M2})=12.33\pm0.05$, $I_c({\rm M3}) =
13.61\pm0.05$, $I_c({\rm M6})=13.82\pm0.05$, $I_c({\rm M7})=12.01\pm0.05$. All
these stars are placed in the East direction of VW Vul, so we included more
comparison objects to the sequence. Figure~1 shows the finding chart for the new
reference stars that we have found near VW Vul, numbered from C1 to C6. Table 1
gives the $V$, $R_c$, $I_c$ data of these new reference stars. The first column
gives the number (see Fig.~1), the second and the third columns are the J2000.0
coordinates of the objects, the last column is the number of different nights
each new reference star has been calibrated to give the average values reported
in columns~4--6. All the stars have been observed for a minimum of 15 months to
a maximum of 19 months, so they can be considered stable.

In 2004, VW Vul has been monitored from June 25 to July 23,
for a total of 16 nights (see Figure 2). More observations have been
collected in 2005, from August 14 to November 10,
so the overall database consists of 26 nights for a total of 95
photometric measurements (Table 2). From these data we can see that VW Vul 
varies between $V=13.71\pm0.02$ and $15.77\pm0.03$. 

We know that the UV emission of VW Vul during quiescence is dominated 
by the accretion disk, plus the white dwarf contribution (Henry \& Sion,
2001; Urban \& Sion, 2006). The strong emission of the disk is evident 
also in the optical $B$ band, with a relatively low difference in the 
average $B-V$ color-index: it varies between $0\fmm30$ during the outburst 
and $0\fmm45$ in quiescence. On the other side, in the infrared part of 
the spectrum, the emission is usually dominated by the late-type 
secondary star. The average value of $V-I_c$ varies between $0\fmm39$ and 
0\fmm65, but the complete variation goes from $V-I_c=0\fmm27$ to $0\fmm79$.

\references

Bruch, A., Engel, A., 1994, {\it A\&AS}, {\bf 104}, 79

Henry, C.K., Sion, E.M., 2001, {\it PASP}, {\bf 113}, 970

Kato, T., 1999, {\it IBVS}, No.\ 4769

Kholopov, P.N., et al., 1985--1990, {\it General Catalogue of Variable 
Stars}, 4th ed., Nauka, Moscow

Landolt, A.U., 1992, {\it AJ}, {\bf 104}, 340

Misselt, K.A., 1996, {\it PASP}, {\bf 108}, 146

Shafter, A.W., 1985, {\it AJ}, {\bf 90}, 643

Spogli, C., Fiorucci, M., Tosti, G., 1998, {\it A\&AS}, {\bf 130}, 485

Stetson, P.B., 1987, {\it PASP}, {\bf 99}, 191

Thorstensen, J.R., Taylor, C.J., Kemp, J., 1998, {\it PASP}, {\bf 110}, 1405

Urban, J.A., Sion, E.M., 2006, {\it ApJ}, {\bf 642}, 1029

Wenzel W, 1985, {\it IBVS}, No.\ 2757

\endreferences

\end{document}
