Alert Notice 569: Monitoring 1SWASP J162117.36+441254.2 for HST observations

February 15, 2017: Dr. Elmé Breedt (University of Warwick) has requested AAVSO observers' assistance in monitoring the eclipsing cataclysmic variable 1SWASP J162117.36+441254.2 (=CSS_J162117.4+441254) beginning now in support of observations scheduled with the Hubble Space Telescope for late February - early March.

Dr. Breedt writes: "1SWASP J162117.36+441254.2 (=CSS1621+4412) is an eclipsing binary star with an orbital period of P = 0.207852 d = 5 h. It was long thought to be a contact binary from the double-hump profile of its optical light curve and broad primary and secondary eclipses, but to our surprise, it went into outburst in June 2016 (Drake et al. 2016 ATEL#9112). Follow-up observations by AAVSO observers and others revealed that the object was actually a cataclysmic variable (Scaringi et al. 2016 ATEL#9122; Thorstensen et al. 2016 ATEL#9141) with a donor star that is unusually hot for its orbital period.

"We have now been awarded time on the Hubble Space Telescope, with which we hope to measure the temperature of the white dwarf in this binary and to look for signs of nuclear evolution of the hot donor star.

"During outburst, this CV becomes bright enough that it could damage the HST detectors, so the AAVSO observations will help to ensure the safety of the instrument. We would like to monitor the brightness of the target in the run-up to the HST observations to ensure that the target is not in outburst and that the HST observations can be executed safely.

"Please monitor 1SWASP J162117.36+441254.2 once per night (V band or clear fliter) from the middle of February, increasing to twice per night in the week before the observations. (At the moment the HST observations are scheduled for sometime between 27 February - 15 March, but an exact date will be assigned and communicated after 16 February). Please report immediately if the target reaches V=14.5 or brighter, as this would indicate that the target is on the rise to outburst.

"Time series observations during the HST observation window would be very welcome as well, if possible. Suggested cadence: 2-3 min."

Observers are requested to begin monitoring this star now and continue until the end of March 2017. Visual observations are also welcome. 1SWASP J162117.36+441254.2 is V~15.6 at minimum and can be as bright as V~13.1 at maximum.

An AAVSO Special Notice will be issued once the exact date of the HST observations is known. Please be sure you have subscribed both to the Alert Notices and the Special Notices.

Coordinates for 1SWASP J162117.36+441254.2:  R.A. 16 21 17.36   Dec. +44 12 54.1   (2000.0)

Charts: Charts with a comparison star sequence for 1SWASP J162117.36+441254.2 may be created using the AAVSO Variable Star Plotter (VSP).

Please use the name 1SWASP J162117.36+441254.2 when submitting observations to the AAVSO International Database.  Report observations as soon after making them as possible, and immediately if the star is seen going into outburst.

This campaign is being monitored on the AAVSO Observing Campaigns webpage and is being followed on the Campaigns and Observation Reports Forum at

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


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