The violet-degraded band system (3400-3700 Å) has been found in the spectrum of a discharge through helium with a trace of YF 3 vapor. The emitter of this system is the hitherto unknown YF + ion. Vibrational analysis of the band system has been carried out. Rotational analysis has shown that the bands are due to the 2Π- 2∆ transition. Molecular orbital configurations which give rise to both participating states are discussed. The 1Σ- X1Σ (0-0) band of YF (λ3572 Å) obtained in absorption has been reanalyzed.