Alert Notice 475: Monitoring of the dwarf nova U Geminorum in support of HST observations

December 22, 2012: As announced by Dr. Matthew Templeton (AAVSO) in AAVSO Special Notice #316 (, Dr. Michael Shara (American Museum of Natural History, Columbia University) has requested dense coverage of the dwarf nova U Gem in support of HST Cosmic Origins Spectrograph (COS) observations scheduled for now through February 20, 2013.

The purpose of the AAVSO observations is twofold. First, they are to provide status information to the HST schedulers so they can be certain U Gem will not be in outburst during the observations. Second, they are to be used for correlation with and interpretation of spectra obtained by HST/COS in an effort to determine the dominant energy source in dwarf novae (and thus the physical mechanism of dwarf novae outbursts), fundamental information still not known about this class of objects (current theories do not explain all of the observational evidence).

Visual observations and multicolor photometry of U Gem are requested from now through March 7, 2013. All observations are welcome to help ensure knowledge of the outburst status of the system. Calibrated UBVRcIc CCD observations are particularly requested; U-band photometry is especially valuable as HST/COS will be observing U Gem in the ultraviolet.

U Gem has nearly returned to minimum following an outburst that began in late November 2012. While it is unlikely to have another outburst until late in this observing season, it is essential to be sure of its status at all times. As of 2012 December 20.4046 UT (JD 2456281.904633) U Gem was at 14.39V (Barbara Harris, New Smyrna Beach, FL, USA) and fading.

Coordinates for U Gem: R.A. 07 55 05.21  Dec. +22 00 04.7 (2000.0)

Charts for U Gem may be created using the AAVSO Variable Star Plotter (VSP,

Please submit observations as soon as possible to the AAVSO International Database using WebObs (see below for link) and using the name U GEM.

This campaign is being followed on the AAVSO Observing Campaigns page (

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


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