Helium contamination from the progenitor stars of planetary nebulae: The   He/H radial gradient and the Delta Y/Delta Z enrichment ratio
  In this work, two aspects of the chemical evolution of 4He in the Galaxy are considered on the basis of a sample of disk planetary nebulae (PN). First, an application of corrections owing to the contamination of 4He from the evolution of the progenitor stars shows that the He/H abundance by number of atoms is reduced by 0.012 to 0.015 in average, leading to an essentially flat He/H radial distribution. Second, a determination of the helium to heavy element enrichment ratio using the same corrections leads to values in the range 2.8 < Delta Y/Delta Z < 3.6 for Yp = 0.23 and 2.0 < Delta Y/Delta Z < 2.8 for Yp = 0.24, in good agreement with recent independent determinations and theoretical models. 
W. J. Maciel