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\IBVShead{5046}{14 March 2001}

\IBVStitle{YY H\lowercase{er} --- Secondary Eclipses in the System Revealed}

\IBVSauth{Hric, L.$^1$; Petr\'ik, K.$^2$; Niarchos, P.$^3$; Veli\v{c}, Z.$^4$; G\'alis, R.$^5$}

\IBVSinst{Astronomical Institute of the Slovak Academy of Sciences,
059~60 Tatransk\'a Lomnica, The Slovak Republic,\\ \indent e-mail: hric@ta3.sk}

\IBVSinst{Department of Physics, Pedagogical Faculty, University of
Trnava, Priemyseln\'a 4, 918~43 Trnava,\\ \indent The Slovak Republic,
e-mail: astropet@ta3.sk}

\IBVSinst{Department of Astrophysics, Astronomy and Mechanics, Faculty
of Physics, University of Athens,\\ \indent 157~84 Zografos, Athens,
Greece}

\IBVSinst{\v{L}. \v{S}t\'{u}ra 16/22-16, 018~61 Belu\v{s}a, The Slovak
Republic}

\IBVSinst{Faculty of Sciences, University of P.J. \v{S}af\'arik,
Moyzesova 16, 041~54 Ko\v{s}ice, The Slovak Republic,\\ \indent e-mail: galis@ta3.sk}

\SIMBADobj{YY Herculis}
\IBVStyp{ * }
\IBVSkey{photometry}
\IBVSabs{The new photometric observational material of YY Her presented}
\IBVSabs{in this paper shows a very pronounced period  of 587.54 days as}
\IBVSabs{well as its half value. Our results revealed the existence of}
\IBVSabs{secondary eclipses of the cool giant.  The new ephemeris of}
\IBVSabs{primary minima is presented:}
\IBVSabs{JD(I,min) = (2450701.6 +/- 1) d + (587.54 +/- 0.5) d * E.}

\begintext

The star YY Her belongs to the classical symbiotic binaries with
nova-like outbursts. It was discovered to be a variable by Wolf (1919).
On the basis of the next observations (Plaut 1932 and B\"{o}hme
1938)~it has been classified as an irregular variable. Finally Herbig
(1950) described the spectrum in detail and identified it as symbiotic.
YY Herculis is a little bit fainter than most of the long-term
monitored symbiotic stars, which is the reason that the historical
light curve has been covered insufficiently and unhomogeneously so far.
The photometric history has been described by Munari et al.\ (1997b).
There have been observed 4 large outbursts and 6 small eruptions since
1890. The last large outburst which appeared in 1993 was studied in
detail by Tatarnikova et al.\ (2000). Munari et al.\ (1997a) and Munari
et al.\ (1997b) on the basis of analysis of all photometric and
spectroscopic data excluded eclipses as the cause of the light
variability.

The new photometric observational material presented in this paper was
performed with the Newton 180/700 telescope equipped with an originally
constructed CCD Camera based on Texas Instruments chip TC 211 and using
$B$ (440 nm), $V$ (540 nm), $R$ (680 nm), and $I$ (825 nm) filters for
modified Johnson--Kron--Cousins system. The magnitudes of the
comparison stars for CCD photometry were obtained through the
calibration on the basis of the standard stars published in Landolt
(1973, 1983, 1992). The observations were secured from July 2, 1995
till November 29, 2000.  All data are collected in Table~1 and are
depicted in Figure~1.  The observational errors do not exceed 0\fmm03
for $B$, 0\fmm02 for $V$, and 0\fmm01 for $R$ and $I$ colours,
respectively.

\IBVSfig{10cm}{5046-f1.eps}{CCD light curves of YY Herculis in $B$,
$V$, $R$, and $I$ colours. Primary and secondary minima are marked}

\IBVSfig{10cm}{5046-f2.eps}{Phase diagram of YY Her in $I$ colour}

\begin{table}[ht]
\normalsize
\begin{center}
\caption{CCD photometric observations of YY Herculis}
\label{yy_bvri_tab}
\begin{tabular}{lcccc|lcccc}
\hline
${\rm JD_{hel}}-2400000$ & $B_{c}$ & $V_{c}$ & $R_{c}$ & $I_{c}$ & ${\rm JD_{hel}}-2400000$ & $B_{c}$ & $V_{c}$ & $R_{c}$ & $I_{c}$ \\
\hline
49899.4157 &  `*' &    12.42  &  11.43 &  10.34 & 51123.2626 & 14.00 &   12.80 &  11.60 &  10.44 \\
49935.3954 &  `*' &    12.43  &  11.43 &  10.36 & 51130.2594 & 13.81 &   12.89 &  11.65 &  10.44 \\
49979.3559 &  `*' &    12.48  &  11.47 &  10.35 & 51250.6112 & 14.46 &   13.37 &  12.02 &  10.65 \\
50000.3220 &  `*' &    12.75  &  11.59 &  10.37 & 51277.5430 & 14.39 &   13.35 &  12.10 &  10.68 \\
50012.3261 &  `*' &    12.78  &  11.63 &  10.42 & 51299.4134 &  `*'  &    `*'  &   `*'  &  10.64 \\
50515.6262 &13.70 &    12.76  &  11.59 &  10.42 & 51304.4071 & 13.99 &   13.33 &  12.01 &  10.64 \\
50579.4392 &13.94 &    12.78  &  11.61 &  10.42 & 51317.4335 & 14.44 &   13.19 &  11.88 &  10.53 \\
50604.4002 &13.99 &    12.78  &  11.66 &  10.45 & 51323.4164 & 14.20 &   13.10 &  11.86 &  10.53 \\
50611.4272 &14.11 &    12.86  &  11.67 &  10.44 & 51355.4073 & 13.93 &   12.96 &  11.78 &  10.48 \\
50639.4201 &14.36 &    13.12  &  11.86 &  10.54 & 51356.3779 & 13.94 &   12.94 &  11.76 &  10.46 \\
50641.4184 &14.26 &    13.04  &  11.84 &  10.50 & 51363.4100 & 14.12 &   12.91 &  11.72 &  10.45 \\
50658.3890 &14.20 &    13.07  &  11.93 &  10.57 & 51394.3846 & 14.20 &   12.91 &  11.69 &  10.46 \\
50672.3839 &14.31 &    13.22  &  11.99 &  10.63 & 51471.3315 & 13.91 &   12.71 &  11.55 &  10.43 \\
50693.3355 &14.34 &    13.35  &  12.02 &  10.65 & 51616.6201 & 13.81 &   12.87 &  11.62 &  10.52 \\
50708.3134 &14.70 &    13.37  &  12.07 &  10.65 & 51643.5527 & 13.83 &   12.75 &  11.56 &  10.48 \\
50720.3281 &14.41 &    13.24  &  12.00 &  10.62 & 51657.4787 & 13.92 &   12.83 &  11.62 &  10.49 \\
50745.2722 &14.04 &    13.13  &  11.86 &  10.50 & 51665.4733 & 13.81 &   12.80 &  11.56 &  10.45 \\
50750.2660 &14.23 &    13.03  &  11.82 &  10.50 & 51698.4447 & 14.08 &   12.83 &  11.67 &  10.48 \\
50773.2227 &14.05 &    13.00  &  11.78 &  10.49 & 51705.4399 & 13.82 &   12.79 &  11.60 &  10.44 \\
50937.4498 &13.76 &    12.72  &  11.52 &  10.49 & 51717.4404 & 14.27 &   12.91 &  11.69 &  10.49 \\
50942.4581 & `*'  &    12.77  &  11.52 &  10.48 & 51731.4458 & 14.13 &   12.84 &  11.69 &  10.50 \\
50972.4199 &13.81 &    12.84  &  11.63 &  10.57 & 51751.4599 & 13.99 &   12.86 &  11.69 &  10.48 \\
50986.4108 &13.98 &    12.88  &  11.65 &  10.59 & 51767.3990 & 13.94 &   12.95 &  11.75 &  10.51 \\
50998.4593 &13.89 &    12.92  &  11.67 &  10.60 & 51778.3671 & 14.17 &   12.99 &  11.76 &  10.51 \\
51028.4116 &13.68 &    12.64  &  11.51 &  10.44 & 51797.3235 & 14.14 &   12.93 &  11.75 &  10.50 \\
51066.3218 &13.82 &    12.65  &  11.48 &  10.42 & 51815.2950 & 14.26 &   13.21 &  11.97 &  10.61 \\
51080.3115 &13.79 &    12.69  &  11.49 &  10.42 & 51817.2966 & 14.18 &   13.24 &  11.97 &  10.62 \\
\hline 
\multicolumn{8}{l}{All data were taken at the Belu\v{s}a Private Observatory}
\end{tabular}
\end{center}
\normalsize
\end{table}

\vskip-22pt
There are three deep minima and one smaller drop (may be two) of the
brightness evident on the light curve in all four colours.  Despite a
certain mismatch between the times of deep minima and ephemeris
published by Munari et al.\ (1997b) the mutual distance of the minima
suggests their connection to the orbital period.  Since the drop of the
deep minima in $B$ colour reaches one magnitude, the heating effect
proposed by Munari et al.\ (1997b) could not be invoked to explain the
variation (Skopal 1996).  Smaller minima, clearly visible at our light
curves (Figure~1) were not mentioned yet in the literature. Therefore
we performed detailed period analysis of all data accessible to us. Our
new data were supplemented by older data secured in the frame of the
international campaign of long-term monitoring of symbiotic stars (Hric
and Skopal 1989) at the Kryonerion, Skalnat\'e Pleso and Wroclaw
observatories. Moreover the $UBV$ photoelectric photometry published by
Munari et al.\ (1997a,b) and Tatarnikova et al.\ (2000) as well as $IR$
photoelectric photometry by Munari et al.\ (1997b) were adopted.

We removed the stages of activity from the data, determined zero points
for particular observatories and detrended the data due to unmonotonous
decline after the outburst in 1993. Such reduced data have undergone in
the detailed period analysis. We found in the data a very pronounced
period around 587~d as well as its half value in all the five colours.
On the basis of the results we can explain the deep minima in the sense
of the eclipses of the hot component by the cool giant.  We tried to
explain the secondary minima by pulsation of the cool giant like in our
previous papers for AG Dra (Friedjung et al.\ 1998, Petr\'{\i}k et
al.\ 1998 and G\'alis et al.\ 1999).  Such explanation is not very
probable for YY Herculis because the secondary minima are relatively
narrow, quite deep and they appeared exactly in the middle between two
primary minima.

The most probable explanation of the secondary minimum is the eclipses
of the cool giant by the circumstellar envelope around the hot
component. When discussing the light curve shape it is necessary to
mention the distinctive features on the smooth light curve running. The
abrupt drop of brightness at orbital phase 0.88 as well as the rise of
brightness at orbital phase 0.12 can be explained by the eclipse of the
white dwarf in the system.  The next drop in brightness at phase
0.94 is probably due to eclipse of the bright spot. The striking rise
of brightness at orbital phase 0.55 and consecutive short-term
variations (flickering) is probably connected with bright spot
appearance.  Moreover we have information about a circular orbit from the
position of secondary minima. For better understanding of the
particular features see Figure~2.

For determination of the times of primary minima we have selected the
best covered minimum in $V$ colour during the period of JD
2450500--2450937. As a result we can present the ephemeris of primary
minima as follows:
$$
{\rm JD(I)_{min}} = 2450701\fday6 \pm 1\day + 587\fday54 \pm 0\fday5 \times E.
$$

We can summarize, that the secondary minima were revealed in this
study and they are prescribed to the eclipses of the cool giant
by the envelope around the hot component. Moreover, the confirmed
existence of the primary minima agrees with the suggestion
mentioned by Tatarnikova et al.\ (2000).

We have to emphasize that for confirmation of our model it is
inevitable to cover the light curve at least in phase interval
0.25--0.65 during this observational season. We would like to call up
an observational campaign dedicated to this object by the method of
photoelectric or CCD photometry in $UBVRI$ colours. In order to cover
the secondary minimum short observations are desirable with one-week
frequency. There is a strong suspicion for the rapid variability
(flickering) after the orbital phase of 0.55 (mid of October 2001).
For the purpose of flickering activity study it is necessary to obtain
few-hours data per night in any colour, most preferably in $U$ and $B$.
Charts with comparison stars are available in electronic form as ps
files upon request (via e-mail) from the authors. We would like to ask the
participants of this campaign to send the observational data in
electronic form in the format of Heliocentric Julian Date and the
corresponding magnitude.

Acknowledgements: This work has been  supported through the Slovak
Academy of Sciences grant No.\ 1008/21 and Pedagogical Faculty, Trnava
University grant No.\ 1/98.

\references

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G\'{a}lis, R., Hric, L., Friedjung, M., Petr\'{\i}k, K., 1999,
{\it Astron.\ Astrophys.}, {\bf 348}, 533

Herbig, G.H., 1950, {\it Publ.\ Astron.\ Soc.\ Pacific}, {\bf 62}, 211

Hric, L., Skopal, A., 1989, {\it Inf.\ Bull.\ Var.\ Stars}, No.\ 3364

Landolt, A.U., 1973, {\it Astron.\ J.}, {\bf 78}, 959

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