Every 5.6 years, an unseen companion crosses in front of a 10.8-magnitude star in the constellation Cepheus, making it dim for a few weeks. The next eclipse of this star, known as EE Cephei, is expected in mid-January 2009. But because these dimmings are leisurely affairs, the American Association of Variable Star Observers (AAVSO) urges experienced amateurs to start monitoring the star right now, and to continue through the end of February.
An interval of years between minima is unusually long for a star system of this type. But more intriguing is the fact that successive fadings of EE Cephei over the past half century have not been carbon copies of one another. Sometimes the star becomes just half as bright as normal, and other times it fades by a factor of five! And the light curve sometimes has kinks in it, hinting the companion may not even be round.
What could the mystery companion be? "It's probably a small, cool star or close binary surrounded by a dust torus, kind of a cosmic doughnut," says the AAVSO's Mike Simonsen. In his blog at http://simostronomy.blogspot.com/2008/12/mystery-star.html, he tells the history of this star and suggests models that could explain the light-curve changes. But there's a chance the light curve in early 2009 will deviate in some new way, upsetting the apple cart again.
EE Cephei lies 1.5° southwest of 4.2-magnitude Epsilon (ε) Cephei, where it marks the easternmost corner of a distinctive trapezoid of 11th-magnitude stars spanning less than 4 arcminutes. The variable lies at right ascension 22h 09m 22.8s, declination +55° 45' 24" (equinox 2000.0).
Simonsen's blog includes a link to AAVSO finder charts for EE Cephei. Once there, enter "EE CEP" (without quotes) in the box for the name of the star.
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Roger W. Sinnott
Senior Contributing Editor
Sky & Telescope