We report the first observation of the decay D+ → ηeνe in two analyses, which combined provide a branching fraction of B(D+ → ηeνe) = (2.16 ± 0.53 ± 0.07) × 10−4. We also provide an improved measurement of B(D+ → ηeνe) = (11.4 ± 0.9 ± 0.4) × 10−4, provide the first form factor measurement, and set the improved upper limit B(D+ → φ eνe) < 0.9× 10−4 (90% C.L.). ∗ Now at: Pacific Northwest National Laboratory, Richland, WA 99352 † Now at: Rutgers University, Piscataway, New Jersey 08855, USA
