  Microquasars are binary star systems with relativistic radio-emitting jets.
They are potential sources of cosmic rays and laboratories for elucidating the
physics of relativistic jets. Here we report the detection of variable
gamma-ray emission above 100 gigaelectron volts from the microquasar LS I +61
303. Six orbital cycles were recorded. Several detections occur at a similar
orbital phase, suggesting the emission is periodic. The strongest gamma-ray
emission is not observed when the two stars are closest to one another,
implying a strong orbital modulation of the emission or the absorption
processes.
