V651 Mon is a binary central star to the bi-polar Planetary Nebula NGC 2346.  The fades are thought to be eclipses of the A5 central star by a dust cloud within the Nebula.  The planetary nebula is visible visually, thus providing an observing challenge when the star becomes active.

 

For an interesting discussion on the variations in V651 Mon, see...

 

'The 1996-1997 fading of V651 Mon, the Binary central star of the Planetary Nebula NGC 2346. Kato et al, 2001