Alert Notice 596: GW Lib time set for second HST observation

August 31, 2017: Further to AAVSO Alert Notices 591 and 595, Dr. Boris Gaensicke (Warwick University) informs us that the date and time for the second of three HST observations in his campaign on the pulsating white dwarf-containing cataclysmic variable GW Lib has been scheduled for

2017 Sep 6 22:36:04 to Sep 7 02:49:26 UT.

Nightly snapshots are requested now through September 2, and intensive monitoring September 3 through 7, then nightly snapshots again through September 17.

Dr. Gaensicke reminds observers that the HST scheduling team will "...insist on a last quiescent measurement [V or positive visual] within 24h of the beginning of the COS observations, i.e. after UT Sep 5, 2017 22:36."

The third and final HST observation of GW Lib will be around September 13/14, but the exact time is not known yet. The schedule of nightly snapshots following September 7 will be modified to include intensive coverage when the HST time is known.

As reported before, 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 within 24 hours before each HST observing time will be used to make a go/no-go decision; data will be used in the analysis of the resulting data.

In addition to uploading observations to the AAVSO 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.

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.


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