Alert Notice 804: RW Cep dimming underway - observations requested

December 16, 2022

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The bright semiregular variable RW Cep is undergoing an episode of apparent dimming, and observations of all types are requested.

Multicolor photometry (all filters), visual observations, and spectroscopy will be valuable in documenting and interpreting this event. Observations should begin now and continue until further notice. Time series observations are not requested; other photometry and visual observations are requested at a rate of once per week at this time. If a change in cadence is warranted, it will be announced in the forum threads above.

Visual observers need to be aware of the redness of RW Cep and make observations using the quick-glance method, rather than looking at the variable for prolonged periods.

Visual and multicolor photometry observations in the AAVSO International Database since 1935 (uninterrupted visual coverage since December 1950) show previous occasions when RW Cep reached magnitudes similar to its current value: in June 1950 (JD~2433457; first observations reported while dimming was underway), December 1955 (JD~2435440), December 1961 (JD~2437650), December 1969 (JD~2440560), and February 1987 (JD~2447100). In each of those episodes, RW Cep reached visual magnitude ~7.6-7.8.

It is important to note that in 2002, the overall brightness of the star increased by about half a magnitude. In 2021, it returned to its previous approximate level of brightness. The current event appears to be a combination of that shift and the type of minimum occasionally seen in the past.

The accompanying light curves of RW Cep are from the AAVSO International Database and show the complete dataset. The scatter among the visual observations is due substantially to the redness of the variable; different choices of comparison stars used by different organizations also contribute to the scatter. Despite the scatter, the overall behavior of the star is evident. The complete light curve may be inspected using the AAVSO Light Curve Generator ( and using the name RW Cep, leaving the From and To dates blank, and choosing All Bands.

Spectroscopy recommendations provided by AAVSO Spectroscopy Section contact Lauren Herrington include: "Use whatever equipment is available. However, likely to be most valuable would be low to medium resolution (R=100 to R=1000) with full spectral coverage, to capture the slope of the continuum. Since the continuum is being measured, it's important to apply instrument response correction (non-corrected spectra are still useful, but less so).

"Full spectral coverage here means full coverage of the visible region, 4000-7000; coverage of the IR would be a nice bonus, too. The exact wavelengths don't matter too much, since researchers will probably be wanting to look at the TiO bands, which cover the entire spectrum.

"For SNR, higher is better, of course; an observer should probably target SNR>50 for a useful spectrum. As far as cadence goes, 1 spectrum per week would be good."

AAVSO observations show RW Cep has a visual range of ~6.3 to ~7.8. VSX gives its range as 6.0-7.6 V.

ATel #15800 (Vollmann and Sigismondi) also reports on the dimming of RW Cep.

Coordinates (J2000): R.A. 22 23 07.02  Dec. +55 57 47.6 (from VSX page for RW Cep)

Charts with comparison stars for RW Cep may be created using the AAVSO Variable Star Plotter (VSP).

Please report observations to the AAVSO International Database or the AAVSO Spectroscopy Database using the name RW CEP.

       JD 2427850-2435868 (Feb 1938-Jan 1957)

       JD 2435868-2443886 (Jan 1957-Jan 1979)

      JD 2443886-2451904 (Jan 1979-Dec 2000)

      JD 2451904-2459922 (Dec 2000-Dec 2022)

      JD 2451904-2459922 (Dec 2000-Dec 2022), with B data

This Alert Notice was compiled by Elizabeth O. Waagen.


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