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\IBVShead{5119}{18 June 2001}

\IBVStitletl{Detection of Supercycle in BF A\lowercase{ra}: Normal}{ SU UM\lowercase{a}-type Dwarf Nova with the Shortest Supercycle}

\IBVSauth{Kato, Taichi$^1$; Stubbings, Rod$^2$; Pearce, Andrew$^3$; Nelson, Peter$^4$;\\ \indent Monard, Berto$^5$}

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

\IBVSinst{19 Greenland Drive, Drouin 3818, Victoria, Australia,
e-mail: stubbo@qedsystems.com.au}

\IBVSinst{32 Monash Ave, Nedlands, WA 6009, Australia,
e-mail: Andrew.Pearce@worley.com.au}

\IBVSinst{RMB 2493, Ellinbank 3820, Australia,
e-mail: pnelson@dcsi.net.au}

\IBVSinst{PO Box 11426, Tiegerpoort 0056, South Africa,
e-mail: LAGMonar@csir.co.za}

\SIMBADobj{BF Ara}
\IBVStyp{ UGSU }
\IBVSkey{dwarf novae -- photometry}

\begintext

ER UMa stars are a small subgroup of SU UMa-type dwarf novae, which have
extremely short supercycles (the interval between successive superoutbursts)
of 19--50 d (for a review, see Kato et al.\ 1999).  The shortest known
supercycles in ``usual'' SU UMa-type dwarf novae had been 90--130 d (e.g.\
Table 1 in Nogami et al.\ 1997), until the discovery a short supercycle
of 84\fday7 in a normal SU UMa-type dwarf nova, SS UMi (Kato et al.\ 2000).
Although several SU UMa-type dwarf novae have been found to occasionally
exhibit short intervals between successive superoutbursts, only few systems
are known to have intermediate outburst statistics between ER UMa stars
and usual SU UMa-type dwarf novae.  The importance of these intermediate
objects in understanding the nature of ER UMa-type objects, and eventually
the origin of mass-transfer in short-period cataclysmic variables,
was described in Kato et al.\ (2000).

BF Ara is a dwarf nova having a range of variability 13.6--(16.0 p
according to the 4th edition of the General Catalogue of Variable Stars.
The star received attention by the discovery of possible superhumps
with an amplitude of 0\fmm25 by Bruch (1983).  However, the star has
been largely neglected by researchers.  Upon noting the possible
presence of a definite periodicity of occurrence of long, bright outbursts,
we have selected the star as monitoring targets of VSNET Collaboration
(http://www.kusastro.kyoto-u.ac.jp/vsnet/).

Visual observations were done with 32-cm (R.S.), 40-cm (A.P.),
32-cm (P.N.) and 32-cm (B.M.) reflectors.  All observations were done using
photoelectrically calibrated $V$-magnitude comparison stars.  The typical
error of visual estimates was less than 0\fmm2, which does not affect
the following discussion.  The total number of observations between 1997
June 24 and 2001 May 3 was 372.

\IBVSfig{10cm}{5119-f1.ps}{Light curve of BF Ara. Ticks represent epochs
of superoutbursts listed in Table 1}

\vskip-11pt
The overall light curve is presented in Figure 1.  Each filled square
represents single estimates and `$\vee$' sign represents upper limits.
The quasi-periodic occurrence of long, bright outbursts and faint states
associated with brief brightenings is clearly demonstrated.  The behavior
is very reminiscent of that of SS UMi (Kato et al.\ 2000).  Table 1
lists the epochs of long, bright outbursts.  Together with the finding
by Bruch (1983), made at $V=14.2$, which is comparable to observed
magnitudes of these outbursts, these outbursts are most likely considered
as superoutbursts of an SU UMa-type dwarf nova.

\begin{table}
\begin{center}
\caption{Superoutbursts of BF Ara}
\begin{tabular}{cccr@{}l}
\hline
JD start & peak magnitude & \multicolumn3c{duration (d)} \\
\hline
2450722 & 13.8 & & $>10$ \\
2450890 & 14.0 & & $>11$ \\
2450980 & 14.1 & & 13 \\
2451055 & 14.0 & & 17 \\
2451229 & 13.8 & & $>7$ \\
2451301 & 14.3 & & 17 \\
2451465 & 13.9 & & 19 \\
2451640 & 14.0 & & $>11$ \\
2451724 & 14.2 & & $>14$ \\
2451812 & 13.9 & & $>9$ \\
2451975 & 14.2 & & 17 & : \\
\hline
\end{tabular}
\end{center}
\end{table}

Noting that the intervals of these outbursts are close to 83 d or
its multiples, the supercycle was determined as 83\fday4, by assuming the
presence of missed superoutbursts during the unobservable seasons.
All observations are well expressed by this representative supercycle;
Figure 2 presents a folded light curve by this period.  Partly because
of faint outbursts being close to the detection limit, and possibly
because of slight cycle-to-cycle variation, the cycle length of
normal outbursts (between superoutbursts) is slightly harder to detect
than in SS UMi (Kato et al.\ 2000).

\IBVSfig{10cm}{5119-f2.ps}{The 83.4-d supercycle of BF Ara.  Upper limits
are omitted for simplicity}

The present observation suggests that BF Ara is a twin of SS UMi
in its outburst pattern.  The suggested superhump period slightly longer
than $\sim2$ hr (Bruch 1983) is, however, significantly longer than that
of SS UMi (Chen et al.\ 1991; Kato et al.\ 1998), but is close to that of
YZ Cnc, as originally suggested by Bruch (1983).  Since YZ Cnc is another
active SU UMa-type dwarf nova, although its supercycle exceeds 100 d,
the similarity is not surprising.  Detailed observations to determine
the superhump characteristics of BF Ara are strongly encouraged.

\references

Bruch, A., 1983, {\it IBVS}, No.\ 2286

Chen. J.-S., Liu, X.-W., Wei, M.-Z. 1991, {\it A\&A}, {\bf 242}, 397

Kato, T., Lipkin, Y., Retter, A., Leibowitz, E., 1998, {\it IBVS}, No.\ 4602

Kato, T., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., Kunjaya, C.,
1999, {\it Disk Instabilities in Close Binary Systems}, p.\ 45, eds.\
S. Mineshige, J. C. Wheeler (Universal Academy Press, Tokyo)

Kato, T., Hanson, G., Poyner, G., Muyllaert, E., Reszelski, M.,
Dubovsky, P. A., 2000, {\it IBVS}, No.\ 4932

Nogami, D., Masuda, S., Kato, T., 1997, {\it PASP}, {\bf 109}, 1114

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