THE EXTINCTION CONSTANTS FOR GALACTIC PLANETARY-NEBULAE

The extinction constants have been determined from Balmer decrement measurements for over 900 planetary nebulae. The observations have been taken at ESO and at OHP. Comparison with published extinction constants shows that our results from ESO are fairly reliable; those from OHP should be taken with caution. An analysis of the extinction constants derived from the Balmer decrement and from the radio/Hbeta flux ratio indicates that the latter tends to be systematically smaller than the former for increasing extinction. We suggest that the radio measurements of Pottasch's group probably underestimate the radio fluxes, at least for some (faintest) objects.