We present far-UV to optical analyses of four hydrogen deficient central stars of planetary nebulae : BD +30 3639 , NGC 40 , NGC 5315 and NGC 6905 .
Using the radiative transfer code CMFGEN , we determined new physical parameters and chemical abundances for these stars .
The results were analyzed in the context of the [ WR ] \rightarrow PG 1159 evolution via the transformed radius-temperature ( R _ { T } \times T _ { * } ) and HR diagrams .
We found that the use of clumping increases R _ { T } by up to \sim 0.5 dex relative to previous results , whilst our homogeneous models showed systematic changes of \sim 0.1 - 0.2 dex .
NGC 5315 showed itself as an odd object among the previously analyzed central stars .
Its temperature ( \sim 76 kK ) is considerably lower than other early-type [ WR ] stars ( \sim 120 - 150 kK ) .
From our models for NGC 5315 and NGC 6905 , it is unclear if early-type [ WR ] stars have smaller C/He mass ratios than other spectral classes , as claimed in the literature .
In particular , the ratio found for NGC 6905 ( \sim 0.8 ) is in rough agreement with evolutionary calculations , and with values derived for [ WCL ] stars .
We analyzed FUSE spectra of these stars for the first time , and identified phosphorus in the spectra of BD +30 3639 , NGC 40 and NGC 5315 through the doublet transition P v \lambda \lambda 1118 , 1128 ( 3 p ^ { 2 } P ^ { o } -3 s ^ { 2 } S ) .
The iron , silicon , phosphorus , sulfur and neon abundances were analyzed in the context of the nucleosynthesis occurring in previous evolutionary phases .
We found evidence for an iron deficiency in BD +30 3639 and NGC 5315 , and from fits to the Si iv lines we determined a solar silicon abundance for BD +30 3639 and NGC 40 .
For phosphorus , an oversolar abundance in the NGC 5315 model was preferred , while in the other stars a solar phosphorus abundance can not be discarded .
Regarding sulfur , we estimated upper limits for its abundance , since no conspicuous lines can be seen in the observed spectra .
We found that neon is overabundant in BD +30 3639 .
In the other stars , neon is weak or undetectable and upper limits for its abundance were estimated .
Our results are in agreement with theoretical predictions and show the usefulness of [ WR ] stars as testbeds for nucleosynthesis calculations in the AGB and post-AGB phases .