We present VLA H i observations and UH88″ deep optical B - and R -band observations of the IR luminous merger Arp 299 ( = NGC 3690 + IC 694 ) .
These data reveal a gas-rich ( M _ { HI } = 3.3 \times 10 ^ { 9 } M _ { \odot } ) optically faint ( \mu _ { B } \gtrsim 27 mag arcsec ^ { -2 } , \mu _ { R } \gtrsim 26 mag arcsec ^ { -2 } ) tidal tail with a length of over 180 kpc .
The size of this tidal feature necessitates an old interaction age for the merger ( \gtrsim 750 Myr since first periapse ) , which is currently experiencing a very young star burst ( \lesssim 20 Myr ) .
The observations reveal a most remarkable structure within the tidal tail : it appears to be composed of two parallel filaments separated by \sim 20 kpc .
One of the filaments is gas rich with little if any starlight , while the other is gas poor .
We believe that this bifurcation results from a warped disk in one of the progenitors .
The quantities and kinematics of the tidal H i suggest that Arp 299 results from the collision of a retrograde Sab-Sb galaxy ( IC 694 ) and a prograde Sbc-Sc galaxy ( NGC 3690 ) that occurred 750 Myr ago and which will merge into a single object in \sim 60 Myr .
We suggest that the present IR luminous phase in this system is due in part to the retrograde spin of IC 694 .
Finally , we discuss the apparent lack of tidal dwarf galaxies within the tail .