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\IBVShead{4373}{19 September 1996}

\title{Long-Term Behavior of the}
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\title{Eclipsing Binary PX Cephei}

\begintext

PX Cep is an eclipsing binary whose variability was
discovered by Romano (1958a, 1958b) and was given
the preliminary name GR31 (Kholopov 1978).
According to the GCVS its photographic magnitude varies between
12\fm 2  and 13\fm 7. The position of the
variable is given as  21\fh 34\fmm 54\fs  + 65\degr 36'00''
(B1950.0), but at this location no star brighter than
15\fmm\ can be found, neither on the Sonneberg astrograph plates,
nor in the POSS database\footnote{http://arch-http.hq.eso.org/cgi-bin/dss}
(see Figure 1).
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\begin{center}
Figure 1. POSS section containing PX Cep (upper box) and its
position as given in GCVS (lower box). The size
of the field is approximately $13' \times 11'$. North at the top, East to the left.
\end{center}
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Using the discoverer's finding chart, PX Cep was found at the coordinates
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\begin{center}
\begin{tabular}{lllll}
   &    21\fh 34\fmm 51\fss 8 &  +65\degr 38'14'' & (B1950.0)  & \\
or &    21\fh 35\fmm 58\fss 1 &  +65\degr 51'43'' & (J2000.0), &
respectively,  \\
\end{tabular}
\end{center}
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and is identical with GSC\,4261\_1335.
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\centerline{2}
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In Table 1 we summarize the elements for PX Cep as
given in the literature.
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\begin{center}
Table 1. Elements for PX Cep
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\begin{tabular}{lllll}
\hline
epoch (HJD) & period (d) & mag & reference \\
\hline
2436098.33 & unknown & 12.2--13.7 (pg) & GCVS \\
2446270.440 & 3.126993 & 12.24--14.64 (V) & Boninsegna (1987) \\
2446680.402 & 3.126905 & ---  & Borovi\v{c}ka (1995) \\	
\hline
\end{tabular}
\end{center}
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Borovi\v{c}ka (1995) collected all minima given in
the literature. There is a large gap between 1959 and 1985, for
which time interval no observations have been published so far.
Borovi\v{c}ka  found an O$-$C, that suggested a changing period.

In an attempt to fill the gap in the data, PX Cep
was measured on about 600 photographic
plates from the Sonneberg plate collection, taken between
1957 and 1995. For the determination of the target's minima,
Argelander's method was applied
using the comparison stars as given by Romano (1958a, 1958b).
On 48 plates PX Cep was found to be fainter than 13\fm 0
(see Table 2). As can be stated from inspection of the
light curve published by Boninsegna (1987), the star reaches
this magnitude	approximately 0\fd 1 before minimum.
Thus, any detection of the target to be fainter than
13\fm 0 provides the time of minimum within an accuracy
of 0\fd 1, this is $\varphi = \pm 0.031$ (see dotted lines
in Figure~2).

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\begin{center}
Table 2. Times of minima found on Sonneberg plates (HJD)
\begin{tabular}{llll}
 & & & \\
2436814.464 & 2439053.391 & 2441983.345 & 2446270.497 \\
2436839.410 & 2439350.521 & 2442602.497 & 2446320.405 \\
2437577.421 & 2439597.569 & 2442627.483 & 2446714.376 \\
2438243.511 & 2439672.498 & 2443340.472 & 2448093.498 \\
2438268.544 & 2440066.508 & 2444172.261 & 2448096.505 \\
2438290.449 & 2440088.430 & 2444541.293 & 2448512.403 \\
2438315.383 & 2440457.488 & 2444816.464 & 2448534.366 \\
2438587.506 & 2440504.398 & 2444985.236 & 2448537.390 \\
2438709.354 & 2440504.428 & 2445138.501 & 2448559.300 \\
2438709.384 & 2440507.413 & 2445163.439 & 2448559.360 \\
2438753.271 & 2441517.479 & 2445676.273 & 2449250.379 \\
2439031.501 & 2441567.451 & 2445973.341 & 2449688.252 \\
\end{tabular}
\end{center}
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In order to obtain the period, all data points of the
orbital light curve were
analysed using the `Analysis of Variance' algorithm
adopted from Schwarzenberg-Czerny (1989).
The epoch was derived from the resulting O$-$C values.
The following elements were obtained:
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\centerline{Min  =  HJD\,2450163.486 + 3\fd 126959 $\times$ E}
\hspace*{73mm}$\pm$15\ \ \ \ \ \ \ \ \ \ \ $\pm$23
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The O$-$C curve for these elements is given in Figure 2.
Our own data are in agreement with the
assumption of a constant period within the accuracy
of the measurement. The three O$-$C values below the
confidence interval were measured by Romano (1962).
From the information provided in the literature
it cannot be decided with certainty what the reason
for these deviations  might be. Two of these points
obviously lie only on the~shoulder
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\centerline{3}
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Figure 2. O$-$C for new elements. Minima
found on Sonneberg plates are plotted with diamonds, star symbols are the times        of minima as collected by Borovi\v{c}ka (1995). The dotted lines
mark the width of the light curve below 13\fm 0, as described in text.
\end{center}

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\noindent of the minimum with
magnitudes given as 12\fm 8 and 12\fm 7. The third one,
13\fm 3, cannot be explained 
assuming a constant period. Considering only the data
collected by Borovi\v{c}ka, a changing period is indeed
suggested, but there is no
convincing indication for that in the Sonneberg data.


Even if the individual error of one single intensity measurement
derived from a photographic observation is quite large, the sheer
number of our measurements and, moreover, the long time
interval covered enable us to derive rather reliable elements, that
can explain the behavior of PX Cep in the past 40 years
within an accuracy of about 0\fd 1.

The author thanks Constanze la Dous for arranging this
interesting project and Peter Kroll (both Sonneberg
Observatory) for lots of help.

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\beginauthors	
\authorsrightcol
Claus HEERLEIN
Sternwarte Sonneberg
96515 Sonneberg
Germany
ch@sophia.stw.tu-ilmenau.de                           %email
\endauthors
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\references

Boninsegna, R., 1987, {\it Inf. Bull. Var. Stars}, No.\,3048

Borovi\v{c}ka, J. 1995, {\it Contrib. Obs. Brno}, No.\,31

Kholopov, P. N. et al., 1978, {\it Inf. Bull. Var. Stars}, No.\,1414

Romano, G., 1958a, {\it La Specola Ariel di Treviso}, {\bf 14}

Romano, G., 1958b, {\it Coelum}, {\bf 26}, 162

Romano, G., 1962, {\it Publ. Obser. Astronom. Padova}, No.\,125

Schwarzenberg-Czerny, A., 1989, {\it Mon. Not. Astr. Soc.}, {\bf 241},  153

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