The central star of the Planetary Nebula NGC 6537

The fact that Space Telescope WFPC2 images of the planetary nebula NGC 6537 fail to show the central star is used to derive a limit to its magnitude: it is fainter than a magnitude of 22.4 in the visible. This is used to deri ve a lower limit to the temperature of the star. The Zanstra temperature is at least 500 000 K. The Energy Balance temperature is found to be consiste nt with this value, as is the ionization state of the nebula. Assuming a re asonable range of distances for the nebula, the radius of the star can be f ound. It is consistent with the mass-radius relation of a white dwarf of 0. 9 M. or higher.