  VLT images in BVI are used to identify the ionizing source centered on Sersic
13, the largest HII region of the giant nearby galaxy NGC 5128 with log
L_Halpha=39.6 erg/s. This ionizing source turns out to be a close pair of
bright and blue star cluster candidates. Spectroscopy obtained with the
Magellan I telescope confirms that these are massive young clusters physically
associated with the giant HII region Sersic 13. The spectra of both clusters
show prominent Wolf-Rayet type emission features, and prominent lines of HI and
HeI, indicative of a very young age (t ~ few x 10^6 yr). Their luminosities
make each of them at least as luminous as the massive young cluster R136 in 30
Doradus in the LMC, and their individual masses are estimated to be 1-7.5x10^5
M_sun. In addition, the projected separation of the cluster pair is 42 pc. The
measured velocity difference between the clusters is small, \Delta V=49+/-21
km/s, and within 2 sigma of the expected orbital velocity V_orb=5-12 km/s if
they are bound. Dynamical models predict that binary clusters with these
properties would merge in a short timescale of a few orbital periods
(P=20-50x10^6 yr). The discovery of this binary cluster suggests that mergers
of young massive clusters could lead to the formation of the most massive
globular clusters such as omega Cen in our Galaxy and G1 in M31. Alternatively,
if they are not gravitationally bound, these objects would individually evolve
into two normal globular clusters.
