  The effect of the nebula optical depth on the determination of the
temperature (T*) of the central stars in planetary nebulae is discussed. Based
on photoionization models for planetary nebulae with different optical depths,
we show, quantitatively, that the details of the distribution of the H and He
II Zanstra temperatures are mainly explained by an optical depth effect; in
particular, that the discrepancy is larger for low stellar temperatures. The
results also show that for high stellar temperatures the He II Zanstra
temperature underestimates the stellar temperature, even for high optical
depths. The stellar temperature, as well as the optical depth, can be obtained
from a Zanstra temperature ratio (ZR) plot ZR = Tz(He II)/Tz(H) versus Tz(He
II). The effects of departures from a blackbody spectrum, as well as of the He
abundance in the nebulae, are also discussed. For nebulae of very low optical
depth and/or high stellar temperature the distribution ZR versus Tz(He II) only
provides lower limits for T*. In order to obtain better values for the optical
depth and T*, we propose the use of the line intensity ratio He II/He I versus
Tz(He II) diagram.
