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\IBVShead{5120}{18 June 2001}

\IBVStitletl{The Second Supercycle of the}{ Helium ER UM\lowercase{a} Star, CR B\lowercase{oo}}

\IBVSauth{Kato, Taichi$^1$; Reszelski, Maciej$^2$; Poyner, Gary$^3$; Simonsen, Mike$^4$;\\ Muyllaert, Eddy$^5$; Dubovsky, Pavol A.$^6$; M\lowercase{c}Gee, Hazel$^7$; Ripero, Jos\'{e}$^8$;\\ Itoh, Hiroshi$^9$}

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

\IBVSinst{Al.\ 1-go Maja 29/4, 64500 Szamotuly, Poland, e-mail: macres@pro.onet.pl}

\IBVSinst{BAA Variable Star Section, 67 Ellerton Road, Kingstanding,
Birmingham B44 0QE, England,\\ \indent e-mail: gp@star.sr.bham.ac.uk}

\IBVSinst{46394 Roanne Drive Macomb, MI USA 48044, e-mail: mikesimonsen@mindspring.com}

\IBVSinst{VVS Belgium --- Werkgroep Veranderlijke Sterren, Eksterstraat
6, 8400 Oostende, Belgium,\\ \indent e-mail: eddy.muyllaert@ping.be}

\IBVSinst{MEDUZA group, Vedecko-kulturne centrum na Orave, 027 42
Podbiel 194, Slovakia,\\ \indent e-mail: vkco@isternet.sk}

\IBVSinst{Starfield, Dedswell Drive, West Clandon, Guildford, Surrey,
GU4 7TQ, England,\\ \indent e-mail: HazelMcGee@compuserve.com}

\IBVSinst{President of CAA (Centro Astronomico de Avila) and Variable
and SNe Group M 1, Buenavista 7, Ciudad\\ \indent Sto.\ Domingo, 28110
Algete/Madrid, Spain, e-mail: jripero@jet.es}

\IBVSinst{(Variable Star Observers League in Japan) VSOLJ,
Nishiteragata-cho 1001-105, Hachioji, Tokyo 192-0153,\\ \indent Japan,
e-mail: PXB02072@nifty.ne.jp} % Ioh


\SIMBADobj{CR Boo}
\IBVSkey{dwarf novae - photometry}

\begintext

CR Boo is the prototype of the helium dwarf novae (for a review of
helium cataclysmic variables, or AM CVn stars, see Warner 1995), which
show very frequent outbursts and superoutbursts.  The characters of
outbursts in CR Boo is regarded as equivalent to ER UMa stars (for
a review, see Kato et al.\ 1999) in hydrogen-rich cataclysmic variables.
Kato et al.\ (2000) showed that the overall light behavior of CR Boo
is well represented by a supercycle (the recurrence time of superoutbursts)
of 46\fday3.  The shortness of the supercycle qualifies CR Boo as a helium
counterpart to hydrogen-rich ER UMa stars (Kato et al.\ 2000).  The
observed properties are in good agreement with the theoretical light
curve (Tsugawa and Osaki 1997).

\IBVSfig{10cm}{5120-f1.ps}{Light curve of CR Boo}

\vskip-11pt
During the extensive observing campaign by the VSNET Collaboration \\
(http://www.kusastro.kyoto-u.ac.jp/vsnet/), we noticed a significant
change in the outburst pattern in CR Boo.  Figure 1 shows the light
curves drawn from visual observations.  The observations used comparison
stars calibrated in the $V$-band, and the typical error of a single
estimate is $\sim0\fmm2$, which will not affect the following discussion.
There is already evident cyclic variations with a period remarkably
shorter than previously reported.

\IBVSfig{10cm}{5120-f2.ps}{Periodogram of CR Boo}

\vskip-11pt
Figure 2 shows the result of period analysis, using the Phase Dispersion
Minimization (PDM) method (Stellingwerf 1978).  The best period is 14\fday7,
which is remarkably shorter than the 46\fday3 period.  Figure 3 shows
the folded light curve by this period.  The light curve clearly shows
the slowly declining plateau portion between phase 0.0 and 0.6, and
a faint state between 0.6 and 0.8.  The linear decline observed in the
section of the outburst between phase 0.0 and 0.6 closely resembles
superoutburst plateau observed in other ER UMa stars and helium ER UMa
stars.  Thus we have found a second supercycle in CR Boo, with an
extremely high superoutburst duty cycle of 0.6.

\IBVSfig{10cm}{5120-f3.ps}{Folded light curve of CR Boo}

Among hydrogen-rich ER UMa stars, RZ LMi (Robertson et al.\ 1995;
Nogami et al.\ 1995) and DI UMa (Kato et al.\ 1996) have extremely short
supercycles of 19--25 d.  They are sometimes called RZ LMi stars,
because of their peculiar characters.  In hydrogen-rich systems, such
a short supercycle cannot be explained by simply increasing the
mass-transfer rate from the secondary star.  Osaki (1995) proposed that
a low tidal torque by the secondary is responsible for such short
supercycles.  It is not yet clear whether the same argument applies in
helium ER UMa stars.  If the newly discovered supercycle is explained
by a temporary increase of mass-transfer rate, this would provide
an evidence of changing mass-transfer rates in helium ER UMa stars.
Otherwise, the present detection of a new supercycle would provide the
first evidence of an alternation between usual ER UMa-state and peculiar
RZ LMi-state in the same system.

\references

Kato, T., Nogami, D., Baba, H., 1996, {\it PASJ}, {\bf 48}, L93

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)

Nogami, D., Kato, T., Masuda, S., Hiratam, R., 1995, {\it IBVS}, No.\ 4155

Osaki, Y., 1995, {\it PASJ}, {\bf 47}, L25

Robertson, J. W., Honeycutt, R. K., Turner, G. W., 1995, {\it PASP}, {\bf 107}, 443

Stellingwerf, R. F., 1978, {\it ApJ}, {\bf 224}, 953

Tsugawa, M., Osaki, Y., 1997, {\it PASJ}, {\bf 49}, 75

Warner, B., 1995, {\it ApSS}, {\bf 225}, 249

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