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AAVSO ALERT NOTICE 331 (January 10, 2006)


Longtime AAVSO member and colleague Dr. George Wallerstein, University of Washington, has requested our assistance in monitoring the Cepheid variable W Vir in support of his scheduled observing later this month at Apache Point Observatory in Sunspot, New Mexico. Visual and CCD observations are requested. For PEP observers who can reach magnitude 10 or fainter, PEP-V observations are also requested.

Dr. Wallerstein will be observing with the 3.5-meter Astrophysical Research Consortium Telescope for one hour per night for 10 nights, starting January 18. His goal is to observe W Vir from minimum to well after maximum. Visual/CCD observations are essential to know where on the light curve his observations fall.

All observers are asked to make nightly observations from now through at least February 5. This field is a sparse one, and even for visual observers, you will have to stray pretty far to find suitable comparison stars, especially as W Vir approaches maximum brightness. W Vir is a morning object so we need observations around the world to properly cover the light curve.

CCD observers are asked to use a V filter. We suggest offsetting to the west to pick up either one of the 11th magnitude stars, or use an ensemble of the 12-13 magnitude stars for your comparison. The full BVRI sequence and field-calibration file can be found in the chart area of the AAVSO web site. W Vir is a slowly varying star, so one or two observations per night is sufficient. It is quite bright and if possible, observe in more than one filter.

At 10th magnitude, W Vir is faint for PEP observers. If you can go this faint, your nightly V-band data are welcome. You might take a longer series of V,C,K observations to improve the mean magnitude of the observation. The V=9.53 star should be your comparison, and the V=10.04 star would make a reasonable check star.

W Vir (R.A. 13h 26m 01.9931s, Dec. -03o 22' 43.424" (2000.0); range 9.46-10.75V; period 17.2736 days; spectral type F0Ib-G0Ib) is the prototype of the W Vir type of Cepheid variable. W Vir variables (Population II Cepheids) do not follow the same period-luminosity relationship as the classical (delta Cep) Cepheids do; a W Vir Cepheid is about 1.5 magnitudes fainter than a classical Cepheid of the same period. Also, its light curve is different in amplitude and shape, and there are spectral differences. Traditionally, both classical Cepheids and W Vir stars are called Cepheids, but they are distinct groups of entirely different objects in different evolutionary stages. For more information on W Vir Cepheids and classical Cepheids, please see the AAVSO Variable Star of the Season/Month.

Charts: THE AAVSO 'b' SCALE CHART FOR W VIR HAS BEEN WITHDRAWN; PLEASE DO NOT USE IT. New 'c' and 'd' scale charts of W Vir with V sequence are available at

Report Object to the AAVSO as: 1320-02 W VIR


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Many thanks for your valuable astronomical contributions and your efforts.

Good observing!

Elizabeth O. Waagen
Senior Technical Assistant

AAVSO 49 Bay State Rd. Cambridge, MA 02138 617-354-0484