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We investigate the reheating of the very late thermal pulse (VLTP) objectV4334 Sgr (Sakurai's Object) using radio observations from the Very LargeArray, and optical spectra obtained with the Very Large Telescope. We find asudden rise of the radio flux at 5 and 8 GHz - from <= 90 micro-Jy and 80 +/-30 micro-Jy in February 2005 to 320 micro-Jy and 280 micro-Jy in June 2006.Optical line emission is also evolving, but the emission lines are fading. Theoptical line emission and early radio flux are attributed to a fast shock (andnot photoionization as was reported earlier) which occurred around 1998. Thefading is due to post-shock cooling and recombination. The recent rapidincrease in radio flux is evidence for the onset of photoionization of carbonstarting around 2005. The current results indicate an increase in the stellartemperature to 12 kK in 2006. The mass ejected in the VLTP eruption is M_ej >=1e-4 Msol, but could be as high as 1e-2 Msol, depending mainly on the distanceand the clumping factor of the outflow. We derive a distance between 1.8 and 5kpc. A high mass loss could expose the helium layer and yield abundancescompatible with those of [WC] and PG1159 stars.

The onset of photoionization in Sakurai's Object (V4334 Sgr)