Alert Notice 732: Observations requested for eclipsing binary ASASSN-21co

Note: Please continue to observe ASASSN-21co to help build a long-term light curve.  -  Elizabeth O. Waagen, January 3, 2023

March 2, 2021

AAVSO Forum threads (scroll to the bottom of a thread for latest posts):
- Campaigns and Observation Reports:
- Eclipsing Binaries:
- Spectroscopy:

Please subscribe to these threads if you are participating in the campaign so you can be updated by the astronomer and by HQ. Join in the discussion or ask questions there!

Dr. Stella Kafka (AAVSO) requests observers' assistance in monitoring the long period eclipsing binary ASASSN-21co.

ASAS-SN data show that an eclipse started between 2021 February 10 and 12. The ASAS-SN light curve in V and g and relevant photometry may be seen here:

An expanded version of the g light curve and its photometry may be seen here:

Archival data from KELT show a suspected period of 11.9 years, with eclipse duration of ~80 days. ASASSN-21co is V~11.7 and g~12.5 at maximum. Very little else is known about this system.

Observations are requested to begin immediately in order to capture early features of the eclipse. Color photometry (all filters) is requested, as is spectroscopy of all resolutions. Visual observations are welcome.

Please continue to monitor ASASSN-21co at least until after it has returned to maximum. Continued coverage may be requested after the eclipse is over, so please be sure to subscribe to the forums for this campaign to receive updates.

Observations submitted to the AAVSO International Database are as follows:

2021 Mar 01.74370 UT, 10.315 I +-/-0.005 (G. Myers, Hillsborough, CA);
01.74297, 10.320 I +-/-0.005 (Myers);
01.74221, 11.431 R +-/-0.006 (Myers);
01.74183, 11.435 R +-/-0.006 (Myers);
01.74073, 12.344 V +-/-0.004 (Myers);
01.73929, 12.347 V +-/-0.004 (Myers);
01.73769, 14.038 B +-/-0.008 (Myers);
01.73625, 14.032 B +-/-0.008 (Myers);

Coordinates (2000.0):  18 17 51.54 -58 07 49.3  (from VSX page for ASASSN-21co)

Finder charts with comparison stars may be created using the AAVSO Variable Star Plotter (VSP).

Please submit observations using the name ASASSN-21CO as follows:
 - Photometry: Use WebObs to submit to the AAVSO International Database
 - Spectroscopy: Submit to the AAVSO Spectroscopic Database (AVSpec)

This AAVSO Alert Notice was compiled by Elizabeth O. Waagen using material provided by Dr. Kafka.


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


An Alert Notice archive 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: