R Coronae Borealis
R Coronae Borealis stars (or R CrB stars) are highly evolved stars whose light undergoes dramatic dimming events caused by dust around the star. In mid-2007, the class prototype entered one of these dimming events, and has yet to return to its normal brightness. Astronomers use the light curves of R CrB and other stars like it to better understand what happens to stars near the ends of their lives. Although R CrB is very faint in all but the largest telescopes, it is visible in binoculars when it is near maximum brightness.
R CrB remains in its extended faint state as of July 2013. It had a short recovery to V=12.0 in mid-2011, faded to V=14.0 again within a few months, and made another recovery in 2012 before fading yet again. No one has any idea when it will return to its bright state again. It's a great target for visual and CCD observers alike, so please keep it under observation!