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Special Notice #345: MR UMa observations needed now and RZ Leo next HST/COS target

April 2, 2013:  Further to AAVSO Alert Notice 471 ( and AAVSO Special Notice #344 (, HST/COS is scheduled to observe the eclipsing SU UMa-type dwarf nova MR UMa:

MR UMa  2013 April 4 04:12:40 through 08:44:27 UT

MR UMa has returned to minimum after going into outburst in early March. Nevertheless, close monitoring through April 5 is important to be sure it is below the threshold brightness and to have a good magnitude value for correlation with the HST observations. MR UMa is V=16.7 at minimum and can be as bright as V=12.3 at maximum.

After MR UMa, the next target will be the eclipsing SU UMa-type dwarf nova RZ Leo:

RZ Leo  2013 April 8 UT    time to be announced

Giving MR UMa highest priority, please begin covering RZ Leo now. Close monitoring through April 9 is essential. Its last superoutburst occurred in May 2006 when RZ Leo reached visual magnitude 12.5. RZ Leo is V=18.7 at minimum and can be as bright as V=11.1 at maximum.

Observations are crucial beginning now and continuing through one night after the HST observations. About 24-36 hours before the planned observations the HST/COS scheduling team will make the final decision whether to observe each system.

As instructed in AAVSO Alert Notice 471, for CCD observers, simultaneous photometry [shortly before, during, and after the HST observations] would be ideal. For the magnitude estimates, a nightly V measurement from now through the HST observations and a day beyond would be best. If positive visual observations are possible, they are welcome, as are visual fainter-than observations fainter than magnitude 15.0.

Coordinates (2000.0)
MR UMa          R.A. 11 31 22.39   Dec. +43 22 38.5
RZ Leo           R.A. 11 37 22.27   Dec. +01 48 58.6     

Charts may be created using the AAVSO Variable Star Plotter (

Please report observations as soon as possible to the AAVSO International Database using the names MR UMA and RZ LEO, respectively. If an outburst is detected, please report observations immediately.  Many thanks!

This AAVSO Special Notice was compiled by Elizabeth O. Waagen.



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