  After the jet break at $t\sim 1.4$ days, the optical afterglow emission of
the long-short burst GRB 060614 can be divided into two components. One is the
power-law decaying forward shock afterglow emission. The other is an excess of
flux in several multi-band photometric observations, which emerges at $\sim$4
days after the burst, significantly earlier than that observed for a supernova
associated with a long-duration GRB. At $t>13.6$ days, the F814W-band flux
drops faster than $t^{-3.2}$. Moreover, the spectrum of the excess component is
very soft and the luminosity is extremely low. These observed signals are
incompatible with those from weak supernovae, but the ejection of $\sim
0.1~M_\odot$ of $r-$process material from a black hole-neutron star merger, as
recently found in some numerical simulations, can produce it. If this
interpretation is correct, it represents the first time that a multi-epoch/band
lightcurve of a Li-Paczynski macronova (also known as kilonova) has been
obtained and black hole-neutron star mergers are sites of significant
production of $r-$process elements.
