1942 superoutburst of OT J213806.6+261957

ATel #2619; Rene Hudec (Astronomical Institute AS CR Ondrejov)
on 13 May 2010; 22:56 UT
Credential Certification: Rene Hudec (rhudec@asu.cas.cz)

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The optical transient/cataclysmic variable in Pegasus, reported in CBET 2273 and CBET 2275 (designated VSX J213806.5+261957 or OT J213806.6+261957) was detected in the extended study of Sonneberg Observarory Archival Sky Patrol Plates (more than 3000 plates analyzed, taken during the years 1928 to 2004) in another (historical) large optical outburst (peak on 1942 Nov 30) at mag(B)=9.8 (+-0.5). The star was found in a superoutburst from 1942 Nov 30 till 1942 Dec 11, being fainter than mag(B) 13 on 1942 Nov 10 (and before this date) and on 1942 Dec 28 (and after this date). This limits the superoutburst duration to be between 12 and 46 days. The object was found to be fainter than mag(B) 13-14 (limiting magnitude) on all other investigated plates confirming that the superoutbursts of this star are extremely rare. This gives a very long recurrence time of the superoutbursts, about 67 years, however we cannot exclude that another superoutburst occurred which was not covered by Sonneberg plates due to seasonal and/or weather gap and hence escaped detection. The detection of another superoutburst confirms that OT J213806.6+261957 is indeed a WZ Sge type dwarf nova, i.e. a short-period cataclysmic variable with a low accretion rate and very rare and large-amplitude superoutbursts (#ATel 2605).