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\IBVShead{4341}{15 May 1996}

\title{Photoelectric UBV observations of EG Andromedae}

\begintext

One of the patterns of the photometric variability of symbiotic binary 
systems is that, caused by the orbital motion. Orbital variability is 
observed in the EG And system, consisting of
an M2\,III giant and a hot subdwarf. The period is  about  480\fdd\
and UBV measurements have been obtained  practically  at  all  orbital
phases. According to the observations during the last years (Hric et al.
1991, Hric et al. 1993, 1994, Skopal et al. 1995) two
minima are present in the orbital photometric variations 
--- a primary, when the giant is towards the observer and 
a secondary one, remote from the primary approximately at the half 
orbital period. The data scattering is a sign of light fluctuations, 
which become most apparent in the area of the secondary minimum in U 
band (Skopal et al. 1995).

     Two different models are  proposed for an explanation  of  the
variations. According to the first of them, a reflection effect has 
place in this system and the  primary minimum  occurs
when the unheated part of the giant's atmosphere is  oriented  towards
the observer (Munari 1993). The hot component of EG And, however, has 
a low luminosity (Murset et al. 1991) which is insufficient 
(see Belyakina 1970) for the heating of the giant. Moreover, the Lyman 
luminosity is spent entirely for ionization of a portion of the wind 
of the giant and according to the approach of Taylor and Seaquist (1984) 
the boundary between the ionized and the neutral portions is far away 
from its photosphere. Skopal (1995) has critically considered the 
possibility for reflection effect and has shown that the light 
variations of many symbiotic systems with low luminosity hot 
components require an explanation of principle.

     According to the second model (Skopal et al. 1993, Skopal 1995) 
a common envelope, having geometry of the equipotential surface 
containing the Lagrangian point L$_2$, is responsible for the existence 
of the two minima in the orbital light curve. In this model, however, 
the giant's continuum, which is dominant in the BV region (Kenyon 
1986), is not taken into account. This model meets, also, the next 
difficulty. In accordance with Kenyon's (1986) colour -- colour 
diagnostic the UV continuum of EG And does not indicate the presence 
of an optically thick accretion disk, such as that formed as a result 
of mass transfer via L$_1$. That is why the view that the giant in this 
system does not fill its Roche lobe and loses mass by means of stellar wind is 
widely accepted (Oliversen et al. 1985, Vogel 1991, Vogel et al. 1992, 
Vogel 1993, Munari 1993, Tomov 1995). The existence of the envelope 
described is not compatible with the existence of the giant's 
wind, expected in accordance with the theory of stellar evolution. 
Having in mind the difficulties of the interpretation as well as the 
light fluctuations pointed we consider that a large amount of 
observational data is required for comparison with the future models.
\vfill \eject

\centerline{2}
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\begin{center}
Table 1. Differential photometry of EG And against HD\,3914, in magnitudes
\end{center}
\vskip 1mm
\begin{center}
\begin{tabular}{|c|c|c|c|c|c|c|c|c|}
\hline
Julian day & $\Delta$~V & $\Delta$~B & $\Delta$~U &    & Julian day
                   & $\Delta$~V & $\Delta$~B & $\Delta$~U \\
\hline
2448582.354 & 0.07 & 1.25 & 2.79 & & 2449971.395 & 0.13 & 1.34 & 2.70  \\
2448609.229 & 0.02 & 1.26 & 2.77 & & 2449973.390 & 0.14 & 1.37 & 2.87  \\
2448636.260 & 0.08 & 1.26 & 2.86 & & 2449988.450 & 0.19 & 1.45 & 3.06  \\
2449046.240 & 0.11 & ---  & ---  & & 2449993.401 & 0.12 & 1.37 & 3.00  \\
2449173.561 & 0.11 & 1.33 & 3.11 & & 2449996.393 & 0.11 & 1.34 & 2.92  \\
2449178.572 & 0.14 & 1.37 & 3.19 & & 2450012.291 & 0.16 & 1.40 & 3.02  \\
2449248.454 & 0.16 & 1.41 & 3.32 & & 2450024.301 & 0.09 & 1.29 & 2.95  \\
2449340.196 & 0.10 & 1.32 & 3.12 & & 2450049.326 & 0.09 & 1.31 & 2.95  \\
2449573.512 & 0.07 & 1.26 & 2.84 & & 2450092.210 & 0.05 & 1.27 & 2.96  \\
2449598.500 & 0.12 & 1.34 & 2.81 & &             &      &      &       \\
\hline
\end{tabular}
\end{center}
\epsfysize=14cm
\centerline{\epsffile{4341-f1.ps}}
\centerline{Figure 1. Orbital UBV light variations of the EG And system}
\vfill \eject

\centerline{3}
\vskip 1mm

     Here we present  nineteen  photoelectric estimates in the three
colour UBV system, obtained after November 1991. They were carried out
with  a  single  channel  photoelectric  photometer,  mounted  at  the
Cassegrain focus of the 0.6~m telescope of the  National  Astronomical
Observatory ``Rozhen". The data on JD\,2450012.291 have been taken using
the similar telescope and equipment of the Astronomical Observatory
Belogradtchik. The star HD\,3914 (${\rm V}=7.0$, ${\rm B}-{\rm V}=0.44$)
has been used as the comparison star. The data derived has been
processed by means of a photoelectric data software (Kirov et al., 1991),
using the reduction coefficients (Zamanov, private communication)
for both atmospheric extinction and standard UBV system. The differential
photometry $\Delta$$m = m({\rm EG\,And})-m({\rm HD\,3914})$ in the three 
colours is presented in Table 1.



Our photometric data indicate orbital variations (Figure 1). The phases have
been recko\-ned using the elements JD$_{\rm min} = 2446336.7+482.2 \times 
{\rm E}$ where the zero phase is at the epoch of the primary minimum 
(Skopal et al., 1993). The kind of the variations detected as well as 
the amplitudes in the different bands are in agreement with the 
results of the other observers (Hric et al. 1991, 1993, 
1994, Skopal et al. 1992, 1995), although 
the phase range of the secondary minimum has not well been covered.


     The authors wish to thank R. Zamanov for obtaining a part of
the observations. This work was supported in part by Bulgarian National
Scientific Foundation grant under contract F - 466/94 with 
Ministry of  Education, Science and Technology.

\vskip 1cm

\beginauthors
\authorsrightcol
Nikolai TOMOV
Mima TOMOVA
NAO ``Rozhen"
P.O.Box 136
BG-4700 Smolyan, Bulgaria
e-mail: rozhen@tu-plovdiv.bg 
\endauthors

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\centerline{4}
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