Alert Notice 506: Alpha Com eclipse observing campaign



December 5, 2014: Drs. Matthew W. Muterspaugh and Gregory W. Henry (Tennessee State University) have requested AAVSO observers' assistance in monitoring the possible Algol-like binary star alpha Com before, during, and after the eclipse they have predicted for mid-to-late January 2015.

The PIs will be using the Fairborn Observatory automated photometric telescopes on Mt. Hopkins in Arizona to monitor the event, but they are requesting additional observations not only to supplement theirs but also specifically to have good coverage in the event of bad weather at Mt. Hopkins.

Alf Com has a 26-year period and is 4.32V at maximum. The expected amplitude of the eclipse is about 0.8 magnitude and the duration somewhere between 28 and 45 hours. The eclipse is predicted for January 25 +/- three days (JD 2457047 +/- 3). In addition to the 26-year binary period, alf Com itself is slightly variable, ranging over a few mmag from night to night and from year to year with no apparent periodicity.

Muterspaugh at al. first calculated the possibility of a 2015 eclipse in 2010 (Astronomical Journal reference Muterspaugh and Henry recently published an article with updated information (

V and/or R photometry (other bands welcome) is requested, starting now. As far in advance of the window as your schedule permits and until the eclipse begins, please obtain a minimum of one measurement/2 hours, to make sure the eclipse is caught right as it begins. (This also will ensure having multiple data points on any unseen objects that might eclipse as well.) During eclipse, continuous observations are important. If you have infrared access (H-band), observations could be interesting, should the CHARA array image the event.

The precision photometry in the weeks before and after the eclipse is requested in order to catch possible evidence for planets or other materials around the eclipsing star.

Regarding refining the predicted time of eclipse, Muterspaugh and Henry write, "Updated astrometry from the current epoch would greatly improve the eclipse preductions. For example, a single measurement with mas astrometric precision from early December 2014, when the projected separation is 36 milli-arcseconds, would improve the timing prediction by a factor of 3."

Coordinates:  RA 13 09 59.28  Dec. +17 31 46.0    (2000)

Charts with comparison star sequence for alf Com may be created using the AAVSO Variable Star Plotter (

The PIs recommend using the comparison stars:
HD 113848 (39 Com, HR 4946, HIP 63948, V=5.990, B-V=0.39, F4V)
HD 114520 (HIP 64312, V=6.820, B-V=0.46,F2II)
They comment that "Both comparison stars are constant to one millimag (0.001 mag) or so on night-to-night and year-to-year timescales; the standard deviation between comparison stars for the complete 22 year dataset with over 2000 observations is only 0.0018 mag."

On the AAVSO A-scale chart for alf Com, HD 113848 is labeled comp 60, and HD 114520 is comp 68. In the photometry table for this star, HD 113848 is 000-BBT-464 and HD 114520 AUID 000-BLL-255.

Please submit observations to the AAVSO International Database using the name ALF COM. Muterspaugh notes that January 23 is the birthday of his sister, Jessica, and if the eclipse occurs on the 23rd, he would like to call it "Jessica's eclipse". If the eclipse does occur on January 23, a note could be added to the WebObs Comments field to this effect.

Thank you very much for your contributions to this campaign.

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


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