Alert Notice 501: Monitoring requested for HST/COS observations of SBS 1108+574

April 29, 2014: Dr. Peter Garnavich (University of Notre Dame) and Dr. Paula Szkody (University of Washington) have requested the help of AAVSO observers in monitoring the cataclysmic variable SBS 1108+574 (= CSS 120422:111127+571239) in support of upcoming Hubble Space Telescope observations. The HST COS (Cosmic Origins Spectrograph) will be carrying out far-ultraviolet spectroscopy of this target on 2014 May 12-13 UT.

SBS 1108+574 is of interest because, Dr. Garnavich writes, "This CV was found in outburst by the Catalina Sky Survey and was found to have an orbital period of only 56 minutes. With this period, we expected to find a helium-rich AM CVn star, but our spectra showed both helium and hydrogen lines."

The primary purpose of the AAVSO monitoring is to know whether SBS 1108+574 is in quiescence immediately prior to the observation window; if it is in outburst it will be too bright for the HST instrumentation. Based on your observations, the HST scheduling team will make the decision about 24 hours before the scheduled observing time as to whether to go forward with the HST observations. Thus, as in other campaigns similar to this one, your observations will be crucial.

The exact time of the HST observations will be announced closer to the date in an AAVSO Special Notice. During quiescence SBS 1108+574 is around magnitude V=18 and around V=15 in outburst.

Nightly V snapshots are requested from now through May 25. More frequent observations are requested May 10-13 (2-3 per night). Simultaneous photometry/time series is not needed. Visual positive observations and fainter-than observations fainter than 15.0 are welcome.

Coordinates:  R.A. 11 11 26.87  Dec. +57 12 38.9  (2000.0)

Charts for SBS 1108+574 may be created using the AAVSO Variable Star Plotter (VSP) at Photometrists should use the data table supplied in VSP.

Please submit your observations to the AAVSO International Database as soon after making them as possible. If you see the variable in outburst, please contact the AAVSO immediately and post a message to the Observations and Campaigns & Observations Reports forum (

The primary name for this object in VSX is CSS 120422:111127+571239. When submitting your observations, please use either this name or the name SBS 1108+574 [be sure to include the space between the two parts of the name].

Your observations and timely reporting are essential. Many thanks for your contributions!

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


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