BF Ara preprint The following article is accepted for publication as IBVS No. 5119. The figures are available at: http://ftp.kusastro.kyoto-u.ac.jp/pub/vsnet/preprints/BF_Ara/ Regards, Taichi Kato === \documentstyle[twoside, epsf]{article} \input{ibvs2.sty} \begin{document} \IBVShead{xxxx}{xx May 2001} \IBVStitletl{Detection of Supercycle in BF Ara:}{Normal SU UMa-type Dwarf Nova with the Shortest Supercycle} \IBVSauth{Taichi~Kato$^1$, Rod~Stubbings$^2$, Andrew~Pearce$^3$, Peter~Nelson$^4$, Berto~Monard$^5$} \vskip 5mm \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} \IBVSobj{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 84.7 d 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). \vskip 3mm BF Ara is a dwarf nova having a range of variability 13.6 -- (16.0p according to the 4-th edition of the General Catalogue of Variable Stars. The star received attention by the discovery of possible superhumps with an amplitude of 0.25 mag by Bruch (1983). However, the star has been largely neglected by researchers. Upon noting the possible presence of a definite periodicity of occurence of long, bright outbursts, we have selected the star as monitoring targets of VSNET Collaboration (http://vsnet.kusastro.kyoto-u.ac.jp/vsnet/). \vskip 3mm 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 0.2 mag, which does not affect the following discussion. The total number of observations between 1997 June 24 and 2001 May 3 was 372. \IBVSfig{10cm}{fig1.ps}{Light Curve of BF Ara. Ticks represent epochs of superoutbursts listed in Table 1.} \vskip 3mm The overall light curve is presented in Figure 1. Each filled square represents single estimates and ``v" 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} Table 1. Superoutbursts of BF Ara \\ \vspace{10pt} \begin{tabular}{ccc} \hline JD start & peak magnitude & 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} \vskip 3mm Noting that the intervals of these outbursts are close to 83 d or its multiples, the supercycle was determined as 83.4 d, 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{8cm}{fig2.ps}{The 83.4-d supercycle of BF Ara. Upper limits are omitted for simplicity.} \vskip 3mm The present observation suggests that BF Ara is a twin of SS UMi in its outburst pattern. The suggested superhump period slightly longer than $\sim$2 hr (Bruch 1983) is, however, significantly longer than that of SS UMi (Chen et al. 1990; 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, IBVS No. 2286 Chen. J.-S., Liu, X.-W., Wei, M.-Z. 1990, A\&A, 242, 397 Kato, T., Lipkin, Y., Retter, A., Leibowitz, E., 1998, IBVS No. 4602 Kato, T., Nogami, D., Baba, H., Masuda, S., Matsumoto, K., Kunjaya, C., 1999, {\it Disk Instabilities in Close Binary Systems}, p45, eds. S. Mineshige, J. C. Wheeler (Universal Academy Press, Tokyo) Kato, T., Hanson, G., Poyner, G., Muyllaert, E., Reszelski, M., Dubovsky, P. A., 2000, IBVS No. 4932 Nogami, D., Masuda, S., Kato, T., 1997, PASP, 109, 1114 \end{document}