Alert Notice 595: GW Lib time scheduled for first HST observation

August 24, 2017: Further to AAVSO Alert Notice 591, Dr. Boris Gaensicke (Warwick University) informs us that the first of three HST observations in his campaign on the pulsating white dwarf-containing cataclysmic variable GW Lib has been scheduled for

2017 Aug 31 20:52:02 to 2017 Sep 1 02:12:21 UTC.

As reported in Alert Notice 591, for the HST observations, AAVSO observations are essential to know the state of the system in order to ensure that the object is NOT in outburst when observed with HST. AAVSO observations made 24-30 hours before each HST observing time will be used to make a go/no-go decision, and will also be used in the analysis of the resulting data.

The 2nd and 3rd HST observations of GW Lib will likely follow one and two weeks, respectively, after the initial observations and corresponding alert notices will be made as more details become available. 

Dr. Gaensicke requests nightly monitoring of this object beginning now, intensive monitoring August 28-September 1, September 3-6, and September 8-11, and then nightly observations again through 2017 September 17. The windows of intensive observations may change slightly when the exact HST times are known. In addition to uploading the data to the database, Dr. Gaensicke requests  that all observations be sent to him by email at:

Coordinates:   RA: 15 19 55.45   Dec: -25 00 25.30   (J2000.0)

This target is increasingly challenging at this time of year, and your efforts are truly appreciated and are crucial to the HST observations.

GW Lib is faint at quiescence, magnitude ~16.8 visual/unfiltered. If visual observers are unable to detect GW Lib, please report any "fainter-than" estimates using the magnitude of the faintest comparison star magnitude seen. CCD observers are asked to use filters during observations if available; V filter is preferred, but B, Rc, and Ic may also be used. Detection of the variable itself is not required unless you can reach V=16.8 in reasonable time, but please use sufficient exposure to detect at least the V=143 comparison star (RA 15:19:39.54 Dec. -25:00:31.8) or V=147 (RA 15:20:01.73 Dec. -24:58:37.7) with a S/N of 10 and report the observation as a "fainter-than" observation. GW Lib may be as bright as V=8.2 in outburst.

It is very important that a V or visual positive observation (not a fainter-than) is obtained 24-30 hours before each scheduled HST time, if at all possible.

Beginning now, please observe this object as often as possible according to the schedule and using the observing procedure outlined above, and submit data as soon as possible to the AAVSO International Database using the name GW LIB. During the nights of intensive observations as indicated above, prompt submission of your observations will be critical. In the event of an outburst, please contact the AAVSO immediately.

Charts with a comparison star sequence for GW Lib may be plotted using the AAVSO Variable Star Plotter (VSP).

This observing campaign is being followed on the AAVSO Campaigns and Observing Reports online forum at

Thank you for your observing efforts and valuable observations!

This AAVSO Alert Notice was prepared by Elizabeth O. Waagen and Dr. Bert Pablo.


Information on submitting observations to the AAVSO may be found at:


An archive of Alert Notices is available at the following URL:

Subscribing and Unsubscribing may be done at the following URL:


Please support the AAVSO and its mission -- Join or donate today: