[vsnet-alert 374] superoutburst of DI UMa - the most extreme "RZ LMi star"

[Taichi Kato <tkato@kusastro.kyoto-u.ac.jp>]

Dear Colleagues,

     Observations at Ouda Station indicates that DI UMa is currently
undergoing a bright outburst.  Time resolved V-band CCD photometry on
Mar. 25 and 26 revealed superhumps with an amplitude of ~0.2 mag, and
a period of 0.05550 day (=79.9 min), which is the shortest ever observed
superhump period among dwarf novae, breaking the record of LL And
(0.05666 day, Kato et al.), and of well-known WZ Sge and AL Com.
DI UMa is thus established to be a member of ultrashort orbital period
SU UMa-type dwarf novae.

     From the record of CCD observations by M. Iida (vsnet-obs) and our
own observations, a supercycle length of ~22 day is suggested; previous
superoutbursts seem to have occurred around Feb. 11 (Iida, mag ~14.5),
Mar. 3 (Ouda team, mag ~15.1).  From its extremely short superhump period
and supercycle length, together with its small outburst amplitude (~2.5
mag), we consider DI UMa to be the second, but the most extreme, member
of "RZ LMi stars", which seem to comprise a sub-population near the CV
minimum period (cf. Psh of RZ LMi: 0.05946 day), among recently discovered
high mass-transfer-rate SU UMa systems, called ER UMa stars (Kato and Kunjaya
1995, Nogami et al. 1995) or RZ LMi stars (Robertson et al. 1995).
Follow-up observations during the current superoutburst are highly welcomed.

Regards,
Taichi Kato and Ouda team

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