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\IBVShead{4388}{31 October 1996}

\title{The period behaviour of BL Eridani}

\begintext

   The ephemeris of BL Eri given in the General Catalogue of Variable
Star (Kholopov et al. 1985) is
\vskip 1mm
\hspace*{25mm}  Min.I = HJD\,2429232.082 + 0\fd 1462 $\times$ E \hfill    (1)
\vskip 1mm

Later, from the first photoelectric observations Kern and Bookmyer (1986)
improved the ephemeris as
\vskip 1mm  
\hspace*{25mm}Min.I = HJD\,2444606.5914 + 0\fd 41696010 $\times$ E \hfill (2)
\vskip 1mm

The orbital period of the system has been discussed in various studies.
Yamasaki et al. (1988) suspected that the orbital period of BL Eri likely
changed between 1980 and 1986. Recently, Qingyao et al. (1996) pointed out
that the orbital period increased, but property of the period changes is
unclear.

   The eclipsing binary BL Eri was observed in January 1996 with the
1.0m telescope at Yunnan Observatory in China. BD$-$12\degr 0814 and BD$-$12\degr 0821
were chosen as the comparison and check stars, respectively. From our observations
two moments of secondary minima were determined, which are listed in Table~1.

   With the ephemeris given by Kern and Bookmyer (1986), we have computed
the (O$-$C)$_1$ values of a number of  times of minima we found in references and have
listed in Table 2. Using these (O$-$C)$_1$ values we derived the O$-$C diagram
displayed in Figure 1. In Figure 1 a systematic trend is seen: the period has been
increasing continuously. In order to determine the rate of period change,
all the minimum times were introduced into a quadratic least squares
solution which resulted in:
\vskip 1mm
\hspace*{5mm}Min.I = HJD\,2444606.5833 + 0\fd 41691786 $\times$ E + 0\fd 000000004286 $\times$E$^2$ \hfill (3)
 \hspace*{53mm}$\pm$3  \hspace*{20mm}$\pm$6  \hspace*{36mm}$\pm$5
\vskip 1mm

\noindent and the rate of period change: dP/dE=0.000000004286 day. With this latter ephemeris
we computed the (O$-$C)$_2$ values in Table 2.  Comparing the (O$-$C)$_1$ values with the (O$-$C)$_2$ values in Table 2 the quadratic ephemeris (3) has smaller deviation
\vskip 3mm
       
\centerline{Table 1. Moments of minimum light of BL Eri}
\vskip 1mm
\begin{center}
\begin{tabular}{cccc}
\hline
      JD(Hel.)   &  Error(day)  &   Min.  &      Filters\\
\hline
\\
    2450096.6707  &  0.0004  &       II   &        B,V\\
    2450099.5889  &  0.0005  &       II   &        B,V\\
\hline
\end{tabular}
\end{center}
\vfill\eject

\centerline{2}
\epsfysize=6cm
\centerline{\epsffile{4388-f1.ps}}
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\begin{center}
Figure 1. The O$-$C diagram plotted using the linear ephemeris (2), the solid curve represents the quadratic fit
\end{center}
\vskip 1mm
\centerline{Table 2}
\vskip 1mm
\normalsize
\begin{center}
\begin{tabular}{crccl}
\hline
  JD(Hel.)  &   E    &     (O$-$C)$_1$ &    (O$-$C)$_2$ &         Source\\
2400000+& & & & \\
\hline
\\
44603.6709  &  $-$7.0    &  $-$0.0018 &    +0.0060 &  Kern and Bookmyer(1986)\\
44604.7146  &  $-$4.5    &  $-$0.0008 &    +0.0075 &   Kern and Bookmyer(1986)\\
44606.5894  &  0.0       &  $-$0.0020   &  +0.0061   & Kern and Bookmyer(1986)\\
44607.6328  &  2.5     &  $-$0.0010   &  +0.0072   & Kern and Bookmyer(1986)\\
45298.8745  &  1660.5  &  $-$0.0791   &  $-$0.0127 &   Yamasaki et al.(1988)\\
45299.9170  &  1663.0  &  $-$0.0790   &  $-$0.0125 &   Yamasaki et al.(1988)\\
45300.9599 &   1665.5  &  $-$0.0785   &  $-$0.0120 &   Yamasaki et al.(1988)\\
48602.1026  &  9582.5  &  $-$0.0090   &  +0.0103   & Qingyao et al.(1994)\\
48603.1452  &  9585.0  &  $-$0.0090   &  +0.0010   & Qingyao et al.(1994)\\
50096.6707  &  13166.5 &  +0.1714     &$-$0.0047   & Present paper\\
50099.5889  &  13173.5 &  +0.1736     &$-$0.0057   & Present paper\\
\hline
\end{tabular}
\end{center}
\large
\vskip 1mm
\noindent
than the linear one (2), this point could also be seen from the sums of
square of the deviations: 
\vskip -.6cm
\begin{displaymath}
\hskip -3cm{\sum_{i} W^2_{{\rm (O-C)_1}_i}=0.07928, \sum_{i}W^2_{{\rm (O-C)_2}_i}=0.00091.}
\end{displaymath}

\vskip .4cm
\beginauthors
\authorsrightcol
QIAN SHENGBANG
LIU QINGYAO
YAN YULAN
GU SHENGHONG
HUANG ZHANQUI
Yunnan Observatory
Chinese Academy of Sciences
Kunming
P.R. China
\endauthors
\vskip -5mm

\references 

Kern, J.R., Bookmyer, B.B., 1986, {\it IBVS}, No.\,2850

Kholopov, P.N. et al., 1985, The General Catalogue of Variable Star 4th
ed. (Nauka, Moscow)

Qingyao Liu et al., 1996, {\it A\&Ap. Suppl. Ser.}, {\bf 118}, 453

Yamasaki, A. et al., 1988, {\it A.J.}, {\bf 95}, 894

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