The 1996-1997 Fading of V651 Monocerotis, the Binary Central Star of the Planetary Nebula NGC 2346

V651 Mon is the binary central star of the bipolar planetary nebula NGC 2346. The star showed the second-ever deep fading in 1996–1997, which was presumably caused by obscuration by a dust cloud in the planetary nebula, as was proposed to explain the 1981–1985 event. The entire duration of the 1996–1997 event was ∼400d, remarkably shorter than the 1981–1985 event, suggesting that the obscuring body was smaller or had a larger tangential velocity. The most remarkable feature in this event was the presence of a sharply defined transient clearing (brightening). From the time-scale of the variation, we propose an upper limit of the projected scale of several times ∼ 10 11 cm of the structure responsible for the brightening. This observation provides the first evidence for a sharply defined,