We present Disco, an asynchronous neighbor discovery and rendezvous protocol that allows two or more nodes operating their radios at low duty cycles (e.g. 1%) to discover and communicate with each other during opportunistic encounters and without any prior synchronization information.??The key challenge is to operate the radio at a low duty cycle but still ensure that discovery is fast, reliable, and predictable over a range of operating conditions.??Disco nodes pick a pair of prime numbers such that the sum of their reciprocals is equal to the desired radio duty cycle.??Each node increments a local counter with a globally-fixed period.??If a node's local counter value is divisible by either of its primes, then the node turns on its radio for one period.??This protocol ensures that two nodes will have some overlapping radio on-time within a bounded number of periods, even if nodes independently set their own duty cycle.