The central stars of He 2-131 and He 2-138 : photometric variations.

The central stars of the planetary nebulae He 2-131 and He 2-138 show variations in their visual magnitudes, with amplitudes of about 0.1 mag. and time scales of a few hours. Both stars were already known to have variable wind features in their spectra. Their photometric and spectroscopic behavior appears to be very similar to that of the prototype 'wind variable' central star, IC 418. We compare this behavior with that exhibited by massive early-type stars, and point out the possible importance of the observed differences as a constraint for the hydrodynamic modeling of hot star winds.