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\IBVShead{4829}{19 January 2000}

\IBVStitletl{Period Changes of the Eclipsing Binary LP Cephei}

\IBVSauth{Wolf, Marek$^1$; \v{S}arounov\'a, Lenka$^2$; Diethelm, Roger$^3$}

\IBVSinst{Astronomical Institute, Charles University Prague, CZ\,--\,180\,00 Praha 8, V Hole\v{s}ovi\v{c}k\'ach 2, Czech Republic \\ \indent e-mail: {\tt wolf@mbox.cesnet.cz}}

\IBVSinst{Astronomical Institute, Czech Academy of Sciences,
CZ\,--\,251\,65 Ond\v{r}ejov, Czech Republic}

\IBVSinst{Astronomical Institute, University Basel,
CH\,--\,4102 Binningen, Venusstrasse 7, Switzerland}

\SIMBADobjAlias{LP Cep}{CSV 5400}

\begintext

The semi-detached eclipsing binary LP Cephei (HBV 484 = SVS 681 = CSV 5400 = P~5577 = GSC~4248.1323; $\alpha_{2000} = 21\hr19\mm50\fsec3$,
$\delta_{2000} = +60\deg42\arcm27\arcs$, Sp.\ F2 + K1, $V_{\rm max} = 12.9$~mag) is a rarely investigated, rather faint binary with a short orbital period of about 0.693 days. This binary was selected as a possible system for the study of apsidal motion (Heged\"us 1988) and thus it was also added to our observational project of eclipsing binaries with eccentric orbit. LP~Cep was discovered to be a variable star photographically by Wachmann (1972), who obtained also the first photographic light curve and determined the first light elements
$$
{\rm Pri.~Min. = HJD~}2430517.4650 + 0.6930642 \times E.
$$
Recently, the first multicolor $UBV(RI)_{\rm C}$ photoelectric observations were carried out by Samec et al.\ (1995, 1997). They derived a photometric solution using the Wilson-Devinney code and concluded that LP~Cep is in a near-contact semidetached configuration consisting of an F2 spectral-type primary and a K1 secondary filling its Roche lobe. They also calculated an improved linear ephemeris
$$
{\rm Pri.~Min. = HJD~}2449621.7322 + 0.69306251 \times E.
$$
Our new CCD photometry of LP~Cep was carried out during the years 1995--1999 at the Ond\v{r}ejov Observatory, Czech Republic. A 65-cm reflecting telescope with a CCD-camera SBIG ST-6 or ST-8 was used. The measurements were done using the standard $R$ filter with typically 60 - 90 s exposure time. The nearby stars GSC 4248.00016 ($V = 13.4 mag$) and GSC 4248.01072 ($V = 12.9 mag$) on the same frame as LP~Cep served as comparison and check stars, respectively. The standard error of measurements varies between 0.01 and 0.02 mag. The new moments of primary minimum and their errors were determined using the least squares fit to the data, by the bisecting cord method and the Kwee--van~Woerden algorithm. Only the lower part of the eclipses was used. These 7 new times of primary minimum are presented in Table~1. In this table, $N$ stands for the number of observations used in the calculation of the minimum time. The epochs were calculated using the linear light elements given by Wachmann (1972).

\begin{table}
\begin{center}
\caption{New precise times of minimum of LP~Cep.}
\begin{tabular}{lclc}
\hline
JD Hel.\,$-$ & \lower6pt\hbox{Epoch} & Error & \lower6pt\hbox{$N$} \\[-3pt]
2400000 & & (days) & \\   
\hline
49999.45478  & 28110.0  &  0.00002 & 69 \\
50261.4348   & 28488.0  &  0.0001  & 38 \\
50261.4366*  & 28488.0  &  0.0019  & 13 \\ 
50525.4934   & 28869.0  &  0.0002  & 29 \\
50945.48889  & 29475.0  &  0.00005 & 38 \\
51442.4113   & 30192.0  &  0.0002  & 34 \\
51535.28135  & 30326.0  &  0.00002 & 67 \\
\hline
\end{tabular}

* published also in Diethelm (1996)
\end{center}
\end{table}

The period change of LP~Cep was studied by means of an $O-C$~diagram analysis. We took into consideration all photoelectric measurements of Samec at al.\ (1997) as well as the original photographic times of minimum obtained by Wachmann (1972). A total of 21 times of minimum light were used in our analysis, with only 2 secondary eclipses among them. The $O-C$ residuals for all times of minimum are shown in Figure~1.  The mean seasonal photographic minima based on the plate archive study presented by Bl\"attler (1996) are also plotted.

The variations of $O-C$ values are remarkable and could 
be caused by a light-time effect. A preliminary analysis of 
the third body orbit gives the following parameters:

\begin{center}
$P_3~{\rm(period)} = 6370 \pm 25$ days, i.e. 17.4 years \\
$T_0~{\rm(time~of~periastron) = J.D.}~2450490 \pm 30$ \\
$A~{\rm(semiamplitude)} = 0.0062 \pm 0.0002$ day \\
$e = 0.42 \pm 0.12$ \\
$\omega = 66\deg\pm 2\deg$ \\
\end{center}

\noindent These values were obtained together with the new linear ephemeris 
$$ \begin{array}{r@{}r@{}l}
{\rm Pri.~Min. = HJD~}24 30517.4655 & {} + 0.69306252 & {} \times E, \\
\pm 0.0003 & \pm 0.00000007 \\
\end{array}
$$
by the least squares method. Assuming a coplanar orbit ($i = 90\deg$) and a total mass of the eclipsing pair with the F2 primary and K1 secondary, $M_1 + M_2 \simeq 2.2~M_{\odot}$ (Harmanec 1988), we can obtain a lower limit for the mass of the third component $M_{3,{\rm min}}$. The value of the mass function $f(M) = 0.0051~M_{\odot}$,
from which the minimum mass of the third body follows as 0.35 $M_{\odot}$. 

Only a small part of the third body period is well-covered by the
precise CCD observations. Moreover, the current light-curve analysis
of Samec et al.\ (1997) gives the third light $L_3 = 0$. Possible third 
component of spectral type M3-M4 with the bolometric magnitude about 
+9.2 mag could be practically invisible in the system with an F2 primary 
($M_{\rm bol} = +3.2$ mag). Therefore, new high-accuracy timings of this 
eclipsing binary are necessary in order to confirm the light-time effect 
in this system. 

\medskip

{\it Acknowledgement.} This work has been supported in part
by the Grant Agency of the Czech Republic, grant No.\ 205-99-0225 
This research has made use of the SIMBAD database, operated at CDS, 
Strasbourg, France.

\newpage

\IBVSfig{9cm}{4829-f1.eps}{$O-C$ residuals for the times of minimum of LP~Cep. The individual photoelectric or our CCD minima are denoted by circles, dots represents the original timings of Wachmann and crosses the mean seasonal minima of Bl\"attler. The curve corresponds to a third body orbit.}

\references

Bl\"attler E., 1996, {\it BBSAG Bull.}, 112, 10

Diethelm R., 1996, {\it BBSAG Bull.}, 112, 3

Harmanec P., 1988, {\it Bull.\ Astr.\ Inst.\ Czech.}, {\bf 39}, 329

Heged\"us T., 1988, {\it Bull.\ Inform.\ CDS}, No. 35, 15

Samec R.G., Carrigan B.J., McDermith R.J., French J., 1995, {\it IBVS}, No.\ 4234

Samec R.G., McDermith R.J., Carrigan B.J., Gray J.D., 1997, {\it AJ}, {\bf 113}, 800

Wachman A.A., 1972, {\it IBVS}, No. 749

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