DY Persei

DY Per is the prototype of what is believed to be a distinct class of variable star.  While they exhibit some properties in common with the R Coronae Borealis stars, their fades occur with great regularity and apparently with the same period as their orbital periods when those are known.  Their light curves are dramatic but different from the RCB stars -- their amplitudes aren't as large, and fades and recoveries are shallower and more symmetric than the RCBs.  They're also a rare class, with only a handful known in the Milky Way, and even then there were only one or two known before automated searches of surveys began uncovering them.