Visual and instrumental observations are important, and "fainter-than" observations are just as critical as positive observations.
The recurrent novae are like classical novae, consisting of a white dwarf primary star accreting mass from a stellar secondary. Accreted material builds up on the white dwarf and eventually reaches the temperature and pressure required for thermonuclear ignition. When it does, the shell of accreted material undergoes thermonuclear fusion, rapidly increasing the system's brightness and ejecting this shell of material off the surface of the white dwarf.