Observations at Ouda Station indicate 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.
Taichi Kato and Ouda team
AAVSO NEWS FLASH No. 8
outburst CCD image by M. Iida (1996 Feb. 11)
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