  In early April 2017, five weeks before passing through perihelion, the
dust-poor comet C/2015 ER61 was observed to undergo a major outburst, during
which its intrinsic brightness increased briefly by about 2 mag. Evidence is
presented in this paper to suggest that the flare-up was in all likelihood a
product of an event of nuclear fragmentation that gave birth to a companion
nucleus, which was first detected more than nine weeks later, on June 11, and
remained under observation for 19 days. The companion is found to have
separated from the parent nucleus with a velocity of less than 1 m/s;
subsequently it was subjected to a nongravitational deceleration of 14 units of
10^{-5} the Sun's gravitational acceleration. The companion's overall
dimensions can from this value be estimated al less than one hundred meters
across. Unlike the primary nucleus, the companion displayed considerable
fluctuations in brightness on time scales spanning at least three orders of
magnitude, from a fraction of one hour to weeks. The long temporal gap between
the companion's birth and first detection appears to be a corollary of the
brightness variability. Evidence from the light curve suggests that the
companion became detectable only because of large amounts of debris that it
started shedding as well as increased activity from area newly exposed by
progressive fragmentation, the same process that also resulted in the
companion's impending disintegration by the end of June of in early July.
