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\begin{document}
\IBVShead{4977}{31 October 2000}

\IBVStitle{BVRI Observations of V516 Cygni in Outburst}

\IBVSauth{Spogli, Corrado$^1$; Fiorucci, Massimo$^1$; Raimondo, Gabriella$^2$}

\IBVSinst{Osservatorio Astronomico, Universit\`a di Perugia, Via A. Pascoli
I-06100 Perugia, Italy}
\IBVSinst{Teramo Astronomical Observatory, Collurania, Teramo, Italy}

\SIMBADobj{V516 Cyg}
\IBVStyp{ CWA }
\IBVSkey{photometry}
\IBVSabs{In this paper we report BVRI observations of the dwarf nova}
\IBVSabs{V516 Cyg during an outburst.}
\IBVSabs{Our data show large loops in the colour-magnitude diagram and we}
\IBVSabs{briefly discuss how this behaviour can be in agreement with disk}
\IBVSabs{instability models.}

\begintext

V516 Cyg is a dwarf nova (DN) that varies between $m_{\rm
pg}\simeq13.8$ and $m_{\rm pg}\simeq16.8$ (Downes \& Shara 1993), but
only a few photometric observations are available in the literature.
We have already analysed this source in Spogli et al.\ (1998), but in
that case we observed the source only during the descending phase after
an outburst. In that period it had colour indices of $B-V=0.1$,
$V-R_c=0.2$, $V-I_c=0.2$ at the apparent maximum.

We observed this DN at the Astronomical Observatory of
Collurania--Teramo during August--September 1998 with the aim to better
evaluate the photometric behaviour during all the outburst phase. The
instruments used and the photometric techniques have been already
described in Spogli et al.\ (2000).

We re-calibrated the comparison stars in the finding chart reported by
Spogli et al.\ (1998). The standard magnitudes are listed in Table 1.
Moreover, we measured the $B$ magnitudes for the stars 2 and~3. 
Although the star~1 is double, we used it as a 
comparison star because the aperture size is sufficiently large to 
collect the light coming from both the sources. Probably one of these
is the radio source WSRTGP 2045+4144 (Taylor et al.\ 1996). 
\vskip 0.5cm

\begin{table}[h]
\caption{$BVR_cI_c$ magnitudes of the selected comparison stars}
\begin{center}
\begin{tabular}{ccccc}
\hline
 No. & $B$ & $V$ & $R_c$ & $I_c$ \\
\hline
 1 & 15.05${}\pm{}$0.06 & 14.20${}\pm{}$0.04 & 13.71${}\pm{}$0.04 & 13.25${}\pm{}$0.04\\
 2 & 16.56${}\pm{}$0.07 & 15.34${}\pm{}$0.04 & 14.72${}\pm{}$0.04 & 14.15${}\pm{}$0.04\\
 3 & 17.07${}\pm{}$0.08 & 15.57${}\pm{}$0.05 & 14.82${}\pm{}$0.04 & 13.91${}\pm{}$0.04\\
\hline
\end{tabular}
\end{center}
\end{table}

Two stars which are very bright in the infrared band are visible in the
field of view, and were used to evaluate colour effects. One star (with
$V = 21.0\pm0.5$, $R_c = 17.2\pm0.1$, $I_c = 13.26\pm0.05$) is situated
$\simeq86\arcm$ E and $\simeq53\arcm$ N with respect to V516 Cyg. The other
($V = 19.2\pm0.3$, $R_c = 16.9\pm0.1$, $I_c = 13.95\pm0.05$) is $\simeq3\arcm$
W and $\simeq26\arcm$ N with respect to the DN\@. We calibrated the
magnitudes also at the Astronomical Observatory of Perugia and no
relevant differences are noticeable.

\IBVSfig{10cm}{4977-f1.ps}{$V$ light curve of V516 Cyg.}

\IBVSfig{10cm}{4977-f2.ps}{Colour index variations of V516 Cyg during the outburst}

\begin{table}
\caption{$BVR_cI_c$ magnitudes of V516 Cyg}
\begin{center}
\begin{tabular}{ccccc}
\hline
JD & \lower6pt\hbox{$B$} & \lower6pt\hbox{$V$} & \lower6pt\hbox{$R_c$} & \lower6pt\hbox{$I_c$} \\[-4pt]
(2451000\,+) & & & & \\
\hline
 52.4202&15.85${}\pm{}$0.03&15.58${}\pm{}$0.04&15.40${}\pm{}$0.02&15.11${}\pm{}$0.03\\
 53.4277&16.74${}\pm{}$0.07&16.36${}\pm{}$0.02&16.10${}\pm{}$0.01&15.66${}\pm{}$0.01\\
 56.3205&17.37${}\pm{}$0.03&16.85${}\pm{}$0.04&16.49${}\pm{}$0.01&15.97${}\pm{}$0.05\\
 56.4615&17.69${}\pm{}$0.02&17.03${}\pm{}$0.04&16.61${}\pm{}$0.02&15.96${}\pm{}$0.04\\
 57.3540&17.53${}\pm{}$0.02&16.93${}\pm{}$0.03&16.54${}\pm{}$0.02&15.92${}\pm{}$0.02\\
 57.5042&17.46${}\pm{}$0.03&16.85${}\pm{}$0.02&16.43${}\pm{}$0.01&15.82${}\pm{}$0.03\\
 58.3344&17.15${}\pm{}$0.04&16.48${}\pm{}$0.04&16.02${}\pm{}$0.01&15.40${}\pm{}$0.02\\
 58.4545&17.08${}\pm{}$0.02&16.40${}\pm{}$0.03&16.03${}\pm{}$0.02&15.49${}\pm{}$0.03\\
 59.3086&15.09${}\pm{}$0.02&14.77${}\pm{}$0.05&14.58${}\pm{}$0.01&14.37${}\pm{}$0.01\\
 59.4285&14.73${}\pm{}$0.01&14.44${}\pm{}$0.05&14.31${}\pm{}$0.01&14.14${}\pm{}$0.02\\
 60.3679&14.23${}\pm{}$0.01&14.00${}\pm{}$0.04&13.89${}\pm{}$0.01&13.82${}\pm{}$0.02\\
 60.4603&14.17${}\pm{}$0.01&13.96${}\pm{}$0.06&13.82${}\pm{}$0.03&13.76${}\pm{}$0.05\\
 63.3205&15.00${}\pm{}$0.03&14.83${}\pm{}$0.01&14.63${}\pm{}$0.01&14.47${}\pm{}$0.03\\
 67.3155&18.00${}\pm{}$0.05&17.42${}\pm{}$0.01&16.86${}\pm{}$0.07&16.41${}\pm{}$0.02\\
\hline
\end{tabular}
\end {center}
\end{table}

\vskip-22pt
The comparison stars were used to find the value of the zero point of
the magnitude scale for each CCD image and, then, to measure the dwarf
nova standard magnitude and its error. Although for many
observational runs we obtained more than one CCD image (usually two in 
every filter), Table 2 reports only the average values. All photometric
$V$ measurements are shown in Figure~1.

At the beginning the dwarf nova was declining from a previous outburst 
and, at the end of August, it reached the minimum values of magnitude: 
$B = 17.7$, $V = 17.0$, $R_c = 16.6$ and $I_c = 16.0$. 
The new rising phase started a few days after and V516 Cyg reached the 
maximum in the night of September 3rd (JD 2451060), with $B = 14.2$, 
$V = 14.0$, $R_c = 13.9$ and $I_c = 13.8$. 
The star returned to minimum on September 10th (JD 2451067) with $V \simeq
17.4$, with a difference between the minimum and the maximum of 
$\Delta V \simeq 3.4$: our data show that sometimes V516 Cyg becomes
fainter than the values listed by many catalogues. In this phase rapid 
oscillations are evident within a range of almost half a magnitude (see
Fig.~1).

Figure 2 shows the almost entire loop of the $B-V$ colour index as a
function of magnitude for V516 Cyg. At similar brightness levels, the
system is bluer during decline and redder at rise.

Only very few investigations of colour changes in DNe during the full
outburst cycle have been carried out up to date. The few observations
reported in literature show that some sources follow a loop in the
colour--magnitude diagram (see Warner 1995). This behaviour can be well
explained with disk instability models. In these models, bursts which
begin at a small disk radius (inside-out) produce symmetric light
curves with comparable rise and decline times, while eruptions starting
at a large disk radius (outside-in) generate asymmetric light curves
with short rise times and protracted declines (Cannizzo \& Kenyon
1987). The colour--magnitude loop can also be used to separate
outside-in and inside-out outburst: in the first case the evolution of
the colours is characterised by a large loop in the C--M diagram, while
the colour evolution is exactly the same in the rise and decline phase
for the inside-out outburst (the loop collapses to a line in the C--M
diagram).

The fact that the outside-in outburst produces a reddening in colour when
the instability sets in is basically equivalent to the UV delay phenomenon
at the onset of outburst (for a review see Osaki 1996). 
In this case we obviously have a large loop in the colour--magnitude 
diagram.

Our data show a large loop in the C--M diagram of V516 Cyg, which
suggests that the predicted outside-in outburst of disk instability models 
are in agreement with the observations presented here.

In conclusion, we can say that the data presented here can be
considered a substantial contribution to the poor optical database of
V516 Cyg. In particular these measurements complete the preceding data
reported in Spogli et al.\ (1998) covering for the first time the
optical multicolour behavior over the outburst cycle.

Our observations are in agreement with outside-in outbursts, as described 
by Cannizzo \& Kenyon (1987) in their disk instability models.
The observed outburst probably starts in the outer part of the disk and 
the heating front propagates from the outside to the inside.

\references

Cannizzo, J.K., Kenyon, S.J., 1987, {\it ApJ}, {\bf 320}, 319
% \BIBCODE{1987ApJ....320...319C}

Downes, R.A., Shara, M.M., 1993, {\it PASP}, {\bf 105}, 127 
% \BIBCODE{1993PASP..105..127D}

Osaki, Y., 1996, {\it PASP}, {\bf 108}, 39 
% \BIBCODE{1996PASP...108...39O}

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

Spogli, C., Fiorucci, M., Raimondo, G., 2000, {\it IBVS}, No.\ 4944 
% \BIBCODE{2000IBVS.4944....1S}

Taylor, A.R., Goss, W.M., Coleman, P.H., et al., 1996, {\it ApJS}, {\bf 107}, 239 
% \BIBCODE{1996ApJS..107..239T}

Warner, B., 1995, ``Cataclysmic Variable Stars'', Cambridge University 
Press, Cambridge % \BIBCODE{1995cvs..book.....W}

\endreferences

\end{document}

