@ R77T, an infra-red (heat-seeking) version. This is thought to use the new Ukrainian MM-2000 seeker although the Russians and Ukrainians are being deliberately vague on this point. This missile could, perhaps, be more useful against stealth aircraft and for "sneak attacks" on an aircraft that does not know it has been targeted. The datalink systems associated with the R77 would allow an aircraft to fire this missile at a target it cannot see or track by pointing it in a direction provided by another aircraft or ground radar. The Russians have traditionally built both radar and infrared versions of the same missile, simplifying spare parts and providing fighter pilots with options in dealing with difficult targets. For example, Russian medium-range tactics call for launching both types of missiles simultaneously to complicate the target's defenses or evasive maneuvers.
@ R77P is a passive-radar missile, designed to be cheaper and pick up radar energy produced by the target. Only the Russians are known to use anti-radiation missiles in the air-to-air role, originally to engage AWACS planes at long ranges. The US is thought to have a secret anti-radiation version if the AIM-120 AMRAAM for similar uses.
@ R77G is a ground-launched version designed to replace the missiles used by the mobile SA-6 system. This uses two solid-fuel boosters which are (basically) R73 (AA-11 Archer) short-range missiles without the heat-seeking warheads. --Stephen V Cole
Russia has been aggressively marketing its R77 (AA-12 Adder) medium-range air-to-air missile, their answer to the American AIM-120 AMRAAM. Three new versions have been offered for export sales to supplement the basic active-radar homing R77A. These are: