Leto is a VSTi emulation of the Synthetone SX1000, a monosynth released by Italian manufacturer Jen in 1977.
Leto’s single oscillator can generate a sawtooth, square or pulse waveform (with adjustable pulsewidth). It has a noise generator enabling you to add white or pink noise to the sound. Two ADSR envelopes control the volume and filter cutoff respectively. A single LFO can modulate the pitch, filter cutoff and pulsewidth simultaneously.
Read more about the SX1000 in the Sound on Sound feature here.