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\IBVShead{4745}{7 August 1999}

\IBVStitletl{CCD PHOTOMETRY OF BF ERIDANI --- }{ A LOW-AMPLITUDE DWARF NOVA?}

\IBVSauth{T.~Kato}

\IBVSinsto{Dept.\ of Astronomy, Kyoto University, Kyoto 606-8502, Japan, e-mail: tkato@kusastro.kyoto-u.ac.jp}

\SIMBADobj{BF Eri}
\IBVStyp{ cataclysmic }
\IBVSkey{Cataclysmic variables - photometry}

\begintext

BF Eri was originally classified as a slowly varying variable star. The fourth edition of General Catalogue of Variable Stars listed it as a semiregular variable star with a range of 13.5-15.5 pg. However, the likely identification of the Einstein survey source 1ES 0437-046 with BF Eri (Elvis et al.\ 1992) and optical spectroscopy (Schachter et al.\ 1996) led to the correct identification as a cataclysmic variable. The source was also detected during the course of the ROSAT All-Sky Survey (1RXS J043929.5-043605, 1RXP J043927-0434.3) and pointed observations (1WGA J0439.4-0433). The ROSAT identification of the Einstein source also led to an identification as a cataclysmic variable of unknown type (Chisholm et al.\ 1999). Independently noting the identification of BF Eri with the ROSAT source, the VSOLJ (Variable Star Observers' League in Japan) and the author started visual and CCD monitoring of this object. The first result of visual monitoring (Watanabe 1999) led to a conclusion that BF Eri is a dwarf nova with a recurrence period of 40--50 d, and the range of 13.2--14.7 m$_v$. I here report long-term and short-term variation by CCD photometry.

The observations were done using an unfiltered ST-7 camera attached
to the Meade 25-cm Schmidt-Cassegrain telescope. The exposure time
was 30 s. The images were dark-subtracted, flat-fielded, and
analyzed using the Java$^{\rm TM}$-based aperture photometry package developed by the author. The differential magnitudes of the variable were measured against GSC 4743.801 (USNO $r$-magnitude 11.7), whose constancy was confirmed by comparison with GSC 4743.797 (USNO $r$-magnitude 12.3).
Table 1 summarizes nightly averaged magnitudes relative to GSC 4743.801.

\begin{table}
\caption{CCD observation of BF Eri }
\begin{center}
\begin{tabular}{ccccc}
\hline
JD start$^a$ & JD end$^a$ & $N^b$ & mag$^c$ & error$^d$ \\
\hline
51196.132 & 51196.198 & 140 & 1.96 & 0.01 \\
51197.080 & 51197.154 & 61 & 1.96 & 0.04 \\
51199.113 & 51199.139 & 20 & 2.05 & 0.03 \\
51201.111 & 51201.137 & 33 & 2.05 & 0.02 \\
51205.064 & 51205.112 & 77 & 2.14 & 0.01 \\
51206.079 & 51206.115 & 36 & 2.00 & 0.03 \\
51207.043 & 51207.099 & 97 & 2.05 & 0.01 \\
51208.048 & 51208.050 & 3 & 1.93 & 0.12 \\
51209.046 & 51209.101 & 118 & 2.20 & 0.01 \\
51210.042 & 51210.069 & 23 & 2.02 & 0.06 \\
51211.038 & 51211.060 & 48 & 2.05 & 0.02 \\
51212.031 & 51212.073 & 82 & 2.34 & 0.02 \\
51214.034 & 51214.035 & 3 & 1.95 & 0.12 \\
51216.021 & 51216.098 & 190 & 2.21 & 0.01 \\
51217.040 & 51217.088 & 117 & 2.19 & 0.01 \\
51218.026 & 51218.083 & 138 & 2.13 & 0.01 \\
51219.014 & 51219.081 & 100 & 1.99 & 0.02 \\
51223.061 & 51223.079 & 11 & 2.17 & 0.14 \\
51224.078 & 51224.082 & 8 & 2.24 & 0.05 \\
51225.076 & 51225.078 & 5 & 1.96 & 0.08 \\
51226.116 & 51226.117 & 2 & 2.20 & 0.17 \\
51229.053 & 51229.055 & 5 & 1.72 & 0.04 \\
51230.087 & 51230.089 & 5 & 1.58 & 0.03 \\
51231.031 & 51231.033 & 6 & 1.70 & 0.07 \\
51231.984 & 51231.985 & 4 & 1.86 & 0.03 \\
51232.931 & 51232.933 & 4 & 2.02 & 0.05 \\
51234.901 & 51234.903 & 5 & 2.49 & 0.04 \\
51236.920 & 51236.920 & 1 & 2.58 & - \\
51237.903 & 51237.905 & 5 & 2.28 & 0.05 \\
51238.909 & 51238.910 & 5 & 2.18 & 0.03 \\
51240.906 & 51240.907 & 5 & 2.25 & 0.10 \\
51242.917 & 51242.918 & 3 & 2.32 & 0.06 \\
51243.906 & 51243.908 & 5 & 2.19 & 0.06 \\
51247.912 & 51247.914 & 5 & 1.96 & 0.05 \\
51248.913 & 51248.915 & 7 & 2.17 & 0.04 \\
51249.910 & 51249.912 & 4 & 2.16 & 0.16 \\
51253.913 & 51253.915 & 5 & 1.99 & 0.09 \\
51260.915 & 51260.917 & 4 & 2.46 & 0.06 \\
\hline
\multicolumn{5}{l}{$^a$ ${\rm JD}-2400000$} \\
\multicolumn{5}{l}{$^b$ Number of frames} \\
\multicolumn{5}{l}{$^c$ Magnitude relative to GSC 4743.801} \\
\multicolumn{5}{l}{$^d$ Standard error of nightly average} \\
\end{tabular}
\end{center}
\end{table}

The long-term light curve of BF Eri is shown in Figure 1. Each point
represents a nightly averaged magnitude, with an error bar indicating
the standard error. The initial part of the light curve corresponds to
the final declining stage of the second outburst reported by Watanabe (1999). Around JD 2451230 (1999 February 20), another distinct brightening (outburst) was observed. The outburst smoothly decayed in the following five days, with an average rate of 0.19 mag\,d$^{-1}$. The combination of overall CCD observation and the visual monitoring (Watanabe 1999) indicates that the typical amplitude of outbursts is $\sim$1 mag. The discovery observation may indicate the amplitude can be as large as 2 mag. The overall behavior of this star resembles those of low-amplitude dwarf novae, or of some NL-type variables. The likely existence of outburst periodicity (Watanabe 1999) prefers the dwarf nova interpretation.

\IBVSfig{10cm}{4745-f1.ps}{Long-term light curve of BF Eri}

\IBVSfig{10cm}{4745-f2.ps}{Short-term variability on 1999 February 6}

\vskip-22pt

Figure 2 illustrates a representative short-term light curve of BF Eri,
where each point represents a single measurement. Slow modulations with
a typical time-scale of 1--2 hours usually exist, with possible superposition of shorter time-scale modulations. However, Fourier analysis of the longer data sets, after subtracting the nightly trend, did not yield
a stable periodicity between 0.05 d and 0.2 d.

This work is partly supported by the Grant-in-Aid for Scientific Research
(10740095) of the Japanese Ministry of Education, Science, Culture, and
Sports.

\references

Chisholm, J. R., Harnden, F, R, Jr., Schachter, J., Micela, G., Sciortino, S.,  Favata, F., 1999, {\it AJ}, {\bf 117}, 1845

Elvis, M., Plummer, D., Schachter, J., Fabbiano, G., 1992, {\it ApJS}, {\bf 80}, 257

Schachter, J., Remillard, R., Saar, S., Favata, F., Sciortino, S., Barbera, M., 1996, {\it ApJ}, {\bf 46}, 747

Watanabe, T., 1999, {\it VSOLJ Variable Star Bulletin}, in press (also available from\\ http://www.kusastro.kyoto-u.ac.jp/vsnet/Mail/vsnet/msg01836.html)

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