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\IBVShead{4489}{6 June 1997}
\centerline{\ft REVEALING OF A NEW SU UMa TYPE VARIABLE STAR}
\centerline{\ft VAR21 CORONAE BOREALIS}

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\begintext

 Var21 Coronae Borealis was discovered and designated by  Antipin
 (1996). He classified this star as a dwarf nova (U
 Geminorum type) with a
 photographic range of 14.5~-~$<$17.5 magnitudes. Several outbursts were
 observed. Two types of them were found,
 differing by duration. The longer outbursts with a duration
 of 15 days or more and the shorter ones lasting about 10
 days.
This behavior is very common to the SU Ursae Majoris subtype of cataclysmic
 variables. These variable stars are characterized by the
 presence of small-amplitude (semi)periodic modulations in the light curve,
 called {\sl superhumps}.
Even though CCD photometry was made by M. Iida (Nagamo, Japan) in 1996 and
T. Vanmunster (Landen, Belgium), no superhumps were
detected.

 \medskip

 During the last superoutburst in May 1997, the CCD photometry at 
Nicholas Copernicus Observatory and Planetarium in Brno (Czech Republic)
was performed on night May 11/12. We used SBIG's CCD camera ST-7 placed in primary
focus of 40cm Newtonian telescope (f=1750mm). For automatic reduction
aperture photometry package MuniPhot (by Filip Hroch, Masaryk
University Brno, using some author's routines), which is based on the well known
DaoPhot II - New generation (Stetson, 1987) was used.

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 Figure 1. Light curve with three superhump maxima. V$-$C is
 difference between variable and comparison star
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 The observing run started on
 JD\,2450580.42 and ended on JD\,2450580.59. Total number of exposures
 was 147 (two of them were omitted from the list beacuse of clouds). The star
 was near to 14.6 mag in R-band
 (Kron-Cousins) filter. Each exposure lasted 90 seconds. All images were
 processed by standard
 dark-frame and flat-field corrections. The standard deviation of magnitude
 of variable
 star
 was $\sigma=0.05$~mag. The star GSC\,2576.2027 close to Var21 CrB
 position was used as a comparison.

 In the  light curve (Figure 1), three superhump maxima
 were definitely detected. Using Phase Dispersion Minimization (Stellingwerf
 1978) analyzing
 routines written by Taichi Kato (Kyoto University, Japan)
 we obtain value of superhump period as
 P$_{SH}$=(0.0743 $\pm$ 0.0006)~day. This result is well within the range of
 superhump periods of usual SU UMa stars, which lies below the lower limit
 of the {\sl period-gap} (Osaki 1996) of cataclysmic variables, which is
 aproximately from 2 hours to 3 hours.

  Independent CCD photometry performed during the same night by 
 Vanmunster (1997) has confirmed our results.
 So there are good reasons to classify variable star Var21~CrB
 as an UGSU (U Geminorum, SU UMa subclass) dwarf nova with a range of
 variability (14.5~-~$<$17.5) mag (P) and coordinates:
 $\alpha_{2000}$ = 16$^{\rm h}$ 00\fmm 03\fss 7 and
 $\delta_{2000}$ = 33\degr 11\farcmm 15\farcss.

 \medskip
 \noindent
 {\it Acknowledgements}: The author would like to thank Taichi Kato (and his group) for valuable
 comments, literature support and providing information, to all
 administrators of VSNET (Variable Stars Network) home page for their work, to Tonny Vanmunster
 for notifying about the outburst and independent observation of superhumps
 confirming  our results, Jan Hollan and Zden{\v e}k
 Mikul{\'a}{\v s}ek for valuable comments and manuscript editing, the
 Masaryk University for Internet access.The
 author is grateful to Filip Hroch for aperture photometry software support.

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\beginauthors
\authorsrightcol
Rudolf NOV{\'A}K
N. Copernicus Observatory
and Planetarium
Krav\'{\i} hora 2
Brno 616 00, Czech Republic
Internet: rudolfn@physics.muni.cz
\endauthors

\references

Antipin, S.V., 1996, {\it IBVS}, No.\,4343

Kato, T., Nogami, D., Baba, H., {\it http://www.kusastro.kyoto-u.ac.jp/vsnet}

Osaki, Y., 1996, {\it PASP}, {\bf 108}, 39

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

Stetson, P.B., 1987, {\it PASP}, {\bf 99}, 191

Vanmunster, T., 1997, private communications and vsnet-alert 896

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