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\IBVShead{4561}{27 February 1998}
 
\IBVStitletl{UBV OBSERVATIONS OF AG D\lowercase{RA}}{ DURING THE 1996--1997 ACTIVE PHASE}

\IBVSauth{M.~TOMOVA, N.~TOMOV}

\IBVSinsto{NAO ``Rozhen", P.O.Box 136, BG-4700 Smolyan, Bulgaria,
          e-mail: rozhen@sm.unacs.bg}

\SIMBADobj{AG Dra}
\IBVStyp{ symbiotic }
\IBVSkey{photometry}

\IBVSabs{UBV photometric data of the symbiotic system AG Dra have been obtained}
\IBVSabs{in 1996 and 1997 during active phase. Two light maxima have been}
\IBVSabs{detected, followed by a decline of the star's brightness almost to}
\IBVSabs{its quiescent magnitude.}
\IBVSabs{It is concluded that the growth of the UBV fluxes results from an} 
\IBVSabs{increase of the emission measure of the nebula, caused by a larger} 
\IBVSabs{production of both the ionizing photons and the hot star wind, which}
\IBVSabs{developed during the outburst.}

\begintext  

The symbiotic system AG Dra has undergone a great number of 
outbursts (Luthardt 1983, Skopal \&\ Chochol 1994, Skopal 1994) and
in many cases has displayed orbital photometric variability during 
the periods between them (Meinunger 1979, Hric et al. 1993). The last 
extended quiescent period was from 1987 to 1993 when the V magnitude 
of the system was about 9\fmm8. This period was followed by a new 
active phase which was characterized by two outbursts in 1994 and 1995 
(Montagni et al. 1996). In the beginning of 1996 the brightness practically 
reached its typical values of the quiescent state, but soon after 
the star entered a phase of increased activity again. The maximum 
values of the brightness, observed by us during this phase, were in 
July 1996 ($\rm V=9\fmm25$) and June 1997 ($\rm V=9\fmm20$), followed by
a rapid decrease almost to the quiescent magnitude.

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\centerline{Table 1. Photometric observations of AG Dra}
\vskip 3mm
\begin{center}
\begin{tabular}{ccrrrccrrr}
\hline
JD$-$2450000&~n~&~~~V~~~&~~~B~~~&~~~U~~~&
JD$-$2450000&~n~&~~~V~~~&~~~B~~~&~~~U~~~\\
\hline
267.3 & 3 & 9.24 & 10.12 & 9.15 & 477.7 & 2 & 9.76 & 11.04 &11.00 \\
268.3 & 3 & 9.26 & 10.14 & 9.21 & 478.7 & 2 & 9.75 & 11.06 &10.99 \\
293.6 & 2 & 9.48 & 10.46 & 9.73 & 504.6 & 3 & 9.72 & 11.01 &10.96 \\
294.6 & 3 & 9.43 & 10.37 & 9.60 & 520.5 & 3 & 9.78 & 11.07 &10.95 \\
295.4 & 3 & 9.41 & 10.35 & 9.47 & 628.5 & 3 & 9.17 &  9.99 & 8.96 \\
296.4 & 3 & 9.38 & 10.32 & 9.45 & 629.4 & 3 & 9.21 & 10.05 & 9.05 \\
297.3 & 3 & 9.41 & 10.31 & 9.42 & 643.5 & 2 & 9.14 & 10.00 & 8.92 \\
303.4 & 2 & 9.34 & 10.27 & 9.36 & 651.3 & 2 & 9.26 & 10.10 & 9.11 \\
317.3 & 3 & 9.48 & 10.50 & 9.47 & 652.3 & 2 & 9.25 & 10.08 & 9.07 \\
321.3 & 3 & 9.58 & 10.68 & 9.83 & 698.3 & 3 & 9.28 & 10.18 & 9.19 \\
322.3 & 3 & 9.60 & 10.68 & 9.84 & 699.3 & 4 & 9.32 & 10.18 & 9.25 \\
324.3 & 2 & 9.60 & 10.69 & 9.85 & 701.3 & 3 & 9.30 & 10.17 & 9.20 \\
390.2 & 2 & 9.77 & 11.05 &11.08 & 702.3 & 3 & 9.28 & 10.17 & 9.19 \\
391.2 & 3 & 9.74 & 11.02 &11.10 & 704.3 & 2 & 9.32 & 10.21 & 9.22 \\
398.2 & 3 & 9.69 & 10.98 &11.06 & 729.2 & 3 & 9.64 & 10.70 &10.17 \\
431.2 & 2 & 9.72 & 11.04 &11.14 & 739.2 & 5 & 9.65 & 10.78 &10.46 \\
466.3 & 2 & 9.72 & 11.01 &---~~ & 741.2 & 2 & 9.71 & 10.83 &10.60 \\
476.7 & 2 & 9.75 & 11.05 &11.00 & 742.2 & 3 & 9.69 & 10.83 &10.58 \\
\hline
\end{tabular}
\end{center}


\IBVSfig{18cm}{4561-f1.ps}{The UBV light curves of AG Dra}


Our observations were performed in the UBV system 
during July 1996 -- October 1997 (Table~1) with a single channel 
photoelectric photometer, mounted at the Cassegrain focus of the
0.6~m telescope of the Na\-ti\-o\-nal Astro\-no\-mi\-cal 
Ob\-ser\-va\-to\-ry ``Ro\-zhen".
The brightness data  in the period JD 2450317$\div$2450324 were obtained 
with the similar telescope and equipment of the Astronomical 
Observatory Belogradtchik. The star BD+67\deg925 having 
$\rm V=9\fmm88$, ${\rm B-V}=0\fmm56$ and
${\rm U-B}=-0\fmm04$ (Skopal \& Chochol 1994) 
was used as a comparison star checked by the star 
BD+67\deg926. We estimated the accuracy using the observational 
data of the check star. The m.s. errors are 
not larger than 0\fmm01 in all bands.

The three colour light curve of AG Dra during the time of the observations 
is shown in Figure 1. Our data cover mainly the declining phases 
and the quiescent period between the two outbursts. It can be 
noted that the light monotonically increases in 
$\rm {JD}\sim2450290\div2450310$, although this interval is a part of the first declining phase.
The brightness of AG Dra varied in all the colours, but the amplitude 
of these variations was largest in U and decreased with the wavelength. 
In the time between the outbursts it reached the typical values of 
the quiescent state. The variation in the U magnitude indicates a mean increase 
for the two light maxima by a factor of about 5.3 of the continuum flux 
compared with the quiescent period before the 1994 outburst. The B and V 
increase factors are equal to 2.3 and 1.6. In our view the reason for 
the visual brightening of AG Dra is the increased radiation of its nebula.
This is directly seen from the energy distribution in the spectrum of AG\,Dra 
from IR to X-ray wavelengths, obtained during quiescence and outburst 
(see Fig. 5 of Greiner et al. 1997). The spectrum in the optical/near-UV 
region is dominated by a nebular continuum which strongly increases during 
the outburst phase. 
Therefore we conclude that the increase of the UBV fluxes results from 
an increase of the emission measure of the nebular component in the system. 
This is probably caused by a larger production of both the ionizing photons 
and the hot star wind, which developed during the outburst (cf. Viotti et al. 
1994). 
Then we can determine the lower limit 
of the contribution of the nebula for the maximal values of the B and V 
light on the basis of the approach used in the work of Montagni et al. (1996), 
using the amplitudes of the variation. It turns out that this lower limit is about  55\% in B and 35\% in V. 

This work was supported in part by Bulgarian National Scientific 
Foundation grant under contract F-466/94 with Ministry of Education and 
Science.

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\references

Greiner, J., Bickert, K., Luthardt, R., Viotti, R., Altamore, A., 
	Gonzalez-Riestra, R., Stencel, R.E., 1997, {\it A\&A}, {\bf 322}, 576 

Hric, L., Skopal, A., Urban, Z., Kom\v{z}\'{\i}k, R., Luthardt, R., 
	Papou\v{s}ek, J., Han\v{z}l, D., Blanco, C., Niarchos, P., Veli\v{c}, 
	Z., Schweitzer, E., 1993, {\it Contr. Astr. Obs. Skalnat\'{e} Pleso}, 
	{\bf 23}, 73

Luthardt, R., 1983, {\it Mitt. Ver\"and. Sterne}, {\bf 9}, 129

Meinunger, L., 1979, {\it IBVS}, No.\,1612

Montagni, F., Maesano, M., Viotti, R., Altamore, A., Tomova, M.,
	Tomov, N., 1996, {\it IBVS}, No.\,4336
	
Skopal, A., 1994, {\it IBVS}, No.\,4096

Skopal, A., \& Chochol D., 1994, {\it IBVS}, No.\,4080

Viotti R., Gonz\'alez-Riestra R., Rossi C., 1994, {\it IAU Circ.},  6039

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