The helium abundance of ejecta from V1500 Cygni (Nova Cygni 1975).

We have studied the He emission spectrum of VI 500 Cygni (Nova Cygni 1975) in the nebular phase of the outburst. The steep Balmer decrement is interpreted as evidence for optical depth in the Balmer series. The observed strength of H was corrected for self-absorption by matching observed and predicted Balmer decrements. He I recombination lines yield the He + abundance. The good agreement between the abundances obtained from triplet (AS 876) and singlet (A6678) lines shows that these lines are unaffected by optical-depth and collisional excitation effects. The He + + abundance was derived from the He II line A54l 2. Our principal result is that the He abundance is identical to that found in planetary nebulae, He/H = 0.11 + 0.01. The He+/H ratio decreases and the He + + /H ratio increases in the interval, but the total abundance He/H is constant to within the errors. This constitutes a check on the analysis. We derive the color temperature of the ionizing radiation field by matching the observed ratio He + + /He +, with the models of Hummer and Mihalas. The color temperature increased from below 5 x l0 K on day 10 to about 1.7 x 106 K by day 300. High temperatures during the late decline are supported by the presence of strong lines of [Ne V] and [Fe VII]. Subject headings: stars: abundances - stars: individual - stars: novae