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\IBVShead{4962}{12 October 2000}

\IBVStitle{New CCD Observations of UU~Sagittae and V477~Lyrae}

\IBVSauth{Kiss, L.L.$^{1,2}$; Kasza, J.$^2$; Borza, S.$^2$}

\IBVSinst{Department of Experimental Physics and Astronomical Observatory,
University of Szeged\\ \indent e-mail: l.kiss@physx.u-szeged.hu}

\IBVSinst{Guest Observer at Konkoly Observatory}


\SIMBADobj{UU Sge}
\SIMBADobj{V477 Lyr}

\IBVStyp{ EA/PN }
\IBVSkey{CCD photometry}
\IBVSabs{New CCD observations of two eclipsing binary planetary}
\IBVSabs{nebula central stars are presented and their recent ephemerides}
\IBVSabs{are discussed.}

\begintext

Eclipsing binary central stars of planetary nebulae form a rare class
of variable stars which are key objects in determining absolute
parameters of central compact systems through light curve modelling.
There are only eight such objects listed in the latest catalogue of
cataclysmic binaries, LMXBs and related objects (Ritter \& Kolb 1998)
and only two of them, UU~Sge (central star of Abell 63) and V477~Lyr
(Abell 46) have detailed analyses in the literature. Earlier
photometric studies of these stars raised the possibility of period
changes that can be attributed to either mass-losing processes or other
kind of interaction between the components (UU~Sge --- Pollacco \& Bell
1993, V477~Lyr --- Pollacco \& Bell 1994). The main aim of this note is
to present new observations which reveal the recent behaviour of the
orbital periods.

Unfiltered CCD photometry was carried out on 5 nights in August, 2000
at the Pisz\-k\'es\-te\-t\H{o} Station of the Konkoly Observatory.  The main
reason for doing unfiltered observations is the relative faintness of
the observed stars ($V_{\rm max} = 14\fmm7$ for UU~Sge, $V_{\rm
max} = 15\fmm1$ for V477~Lyr).  The applied instrument was the
60/90/180-cm Schmidt telescope equipped with a Photometrics AT200 CCD
camera ($1536\times 1024$ pixels, KAF-1600 chip with UV-coating). The
field of view is $29\arcm\times18\arcm$ yielding an angular resolution
of 1\farcs1/pixel. The exposure times varied from 60 sec to 180 sec
depending on the actual weather conditions and target brightness. The
image reduction performed with the standard tasks in IRAF included
flat-fielding with a master frame formed from several individual
exposures taken during the evening twilight. Differential aperture
photometry was made with the IRAF/APPHOT package.  In both cases we
chose two nearby field stars at similar brightnesses as comparison and
check stars. Their magnitude differences were used to estimate the
photometric accuracy which is about $\pm0.01$--0.03 mag, the latter value
being typical during the primary minima (e.g., UU~Sge was only barely
detectable in the primary minimum, see below). Individual data are
available upon request from the first author.

\bigskip

\noindent {\bf UU~Sagittae}

This star was observed on all of the five nights (August 3/4, 4/5, 7/8,
8/9 and 9/10) by obtaining 174 individual CCD frames. There is an
optical companion ($V=15\fmm87$) at a distance of 2\farcs8 toward PA
92$^\circ$ (Ciardullo et al.\ 1999) which strongly reduces the eclipse
depth when including its contribution. Since our image scale
(1\farcs1/pixel) prevented an appropriate removing of the second light
of optical companion, we applied an aperture photometry that included
both stars. The reduced unfiltered eclipse depth was found to be
0\fmm9.  The light curve exhibits a strong reflection effect with an
amplitude of 0\fmm3 (Fig.~1, left panel).

\IBVSfig{6cm}{4962-f1.eps}{{\it Left}: the phase diagram of UU~Sge with the
adopted ephemeris. {\it Right}: The $O-C$ diagram of UU~Sge. The solid
line shows the linear fit, while the dotted line corresponds to the
zero level.}

We have tried to separate the effect of the optical companion by
measuring its brightness relative to the chosen comparison stars during
the primary minimum, when UU~Sge has a minimal contribution.  By
choosing an aperture radius of 2 pixels we could exclude UU~Sge's light
and we calculated the unfiltered magnitude differences between the
comparison stars and optical companion. This value was taken when
correcting the combined differential magnitudes using larger aperture.
After the second light removal we obtained a corrected light curve with
an eclipse depth of $4.5\pm0.3$ mag which is close to the real value
(Bell et al.\ 1994 reported $\Delta V=4.05\pm0.06$ mag with the 4.2-m
William Herschel Telescope). This correction illustrates the definite
detection of UU~Sge even in the primary minimum ($V_{\rm
min}=19.20\pm0.07$ mag, Bell et al.\ 1994). The scatter of the
corrected light curve disabled the accurate eclipse timing.
Fortunately, the primary minimum shows a quite symmetric light curve,
thus the direct measurements could be used for determining epochs of
minimum.

Three new times of minimum were obtained by determining the midpoints
of several (2-3) chords taken a few tenth of a magnitude above the
bottom part of the light curve (Table~1). The estimated timing accuracy
is about 0.0005--0.0007 days (the first epoch has larger uncertainty
because of the partial coverage). These times of minima were added to
the published values collected by Pollacco \& Bell (1993), while a
further epoch was presented by Bell et al.\ (1994). The final sample
contains 33 individual epochs which were used to calculate the $O-C$
diagram covering 76 years. The used ephemeris was the following
(Pollacco \& Bell 1993):
$$
{\rm HJD(Min~I)}=2448133.40747+0.465069102\times E.
$$

Early data are photographic, thus their lower accuracy is the main
reason for the considerable scatter of the first part of the diagram.
The general appearance suggests a slightly larger, but most likely
constant period which was determined by a least-squares linear fit of
the $O-C$ values. The fitted slope is $(1.05\pm0.49)\times10^{-7}$
resulting in a corrected period of 0.46506921(5) days. This is a little
longer period than that of by Bell et al.\ (1994), however, the
difference is very close to the detection limit. Therefore, we
conclude, that our new ephemeris
$$
{\rm HJD(Min~I)}=2451766.5285+0.46506921\times E
$$
is a marginally improved one allowing accurate phase predictions for
the follow-up observations.

\begin{table}
\caption{New times of minima (Hel.~JD${}-2400000$). $O-C$ values
were calculated with the ephemerides given in Pollaco \& Bell (1993) ---
UU~Sge, and Pollacco \& Bell (1994) --- V477~Lyr.}
\vskip 3mm
\begin{center}
\begin{tabular}{lllr}
\hline
& Hel.~JD  &  $E$ &  $O-C$ \\
\hline
UU Sge & 51760.4835: & 7799 & 0.0021 \\
& 51761.4120  & 7801 & 0.0005 \\
&51766.5285  & 7812 & 0.0012 \\
\hline
V477 Lyr & 51764.5163  & 7693 & $-$0.0002 \\
& 51765.4605  & 7695 & 0.0005 \\
& 51766.4038  & 7697 & 0.0004 \\
\hline
\end{tabular}
\end{center}
\end{table}

\bigskip

\noindent {\bf V477~Lyrae}

V477~Lyrae ($V_{\rm max}=15\fmm1$, $\Delta V=1\fmm5$, $P=0\fday4717$)
was observed on three subsequent nights (August 7/8, 8/9 and 9/10), thus
only partial phase coverage could be obtained. Although V477~Lyr is similar
to UU~Sge, there are much less observations in the literature as for UU~Sge.
The most striking feature of the light curve is the strong reflection effect
with an amplitude up to 0\fmm5 (see left panel in Fig.~2).
Pollacco \& Bell (1994) presented the first full light-curve analysis
for V477~Lyr concluding that Abell 46 may be an example of
a `lazy PN' (the central star is visually bright whilst
the nebula itself has a low surface brightness), while the secondary
component is oversized for its mass. There are 12 published
epochs of minimum which were listed by Pollacco \& Bell (1994). Since then
there were no new observations on this star.

\IBVSfig{6cm}{4962-f2.eps}{{\it Left}: the phase diagram of V477~Lyr with the
adopted ephemeris. Data obtained on the first night were excluded because
of larger scatter caused by the unfavorable weather conditions.
{\it Right}: The $O-C$ diagram of V477~Lyr. Obviously inaccurate
photographic data were exluded, while the dotted line corresponds to
the zero level.}

Our observations resulted in three new times of minima (Table~1) which
were determined by fitting low-order polynomials to the lowest part of
the individual light curves. Since the primary minimum is only partial, the
timing is more accurate than in the case of UU~Sge, with an estimated
accuracy of 0.0003 days. The $O-C$ diagram was calculated with the
ephemeris in Pollacco \& Bell (1994):
$$
{\rm HJD(Min~I)}=2448135.50446+0.47172909\times E.
$$

As can be seen in the right panel of Fig.~2, there is no significant
variation in the $O-C$ diagram, which corresponds to a stable period
over 40000 cycles (about 50 years).  A formal linear fit gives a slope
of $(-1.1\pm1.1)\times 10^{-7}$ suggesting a relative stability of the
period $\Delta P/P \approx1.1 \times 10^{-7}/0.4717 \approx 2 \times
10^{-7}$.  Therefore, we conclude that no period change can be detected
using the presently available data and follow-up observations can be
planned with the cited ephemeris.

This research was supported by the ``Bolyai J\'anos'' Research
Scholarship of LLK from the Hungarian Academy of Sciences, Hungarian
OTKA Grant \#T032258 and Szeged Observatory Foundation.  The
observations were acquired during the course of Summer Training
Programme for students of astronomy at University of Szeged.  The warm
hospitality of the staff of the Konkoly Observatory and their provision
of telescope time is gratefully acknowledged.  The NASA ADS Abstract
Service was used to access data and references.  This research has made
use of Simbad Database operated at CDS, Strasbourg, France.

\references

Bell, S.A., Pollacco, D.L., Hilditch, R.W., 1994, {\it MNRAS}, {\bf 270}, 449

Ciardullo, R., Bond, H.E., Sipior M.S. et al, 1999, {\it AJ}, {\bf 118}, 488

Pollacco, D.L., Bell, S.A., 1993, {\it MNRAS}, {\bf 262}, 377

Pollacco, D.L., Bell, S.A., 1994, {\it MNRAS}, {\bf 267}, 452

Ritter, H., Kolb, U., 1998, {\it A\&AS}, {\bf 129}, 83

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