In the course of the MUCHFUSS project we have recently discovered four radial velocity ( RV ) variable , hot ( T _ { \mathrm { eff } } \approx 80 000 - 110 000 K ) post-asymptotic giant branch ( AGB ) stars .
Among them , we found the first known RV variable O ( He ) star , the only second known RV variable PG 1159 close binary candidate , as well as the first two naked ( i.e. , without planetary nebula ( PN ) ) H-rich post-AGB stars of spectral type O ( H ) that show significant RV variations .
We present a non-LTE spectral analysis of these stars along with one further O ( H ) -type star whose RV variations were found to be not significant .
We also report the discovery of an far-infrared excess in the case of the PG 1159 star .
None of the stars in our sample displays nebular emission lines , which can be explained well in terms of a very late thermal pulse evolution in the case of the PG 1159 star .
The “ missing ” PNe around the O ( H ) -type stars seem strange , since we find that several central stars of PNe have much longer post-AGB times .
Besides the non-ejection of a PN , the occurrence of a late thermal pulse , or the re-accretion of the PN in the previous post-AGB evolution offer possible explanations for those stars not harbouring a PN ( anymore ) .
In case of the O ( He ) star J0757 we speculate that it might have been previously part of a compact He transferring binary system .
In this scenario , the mass transfer must have stopped after a certain time , leaving behind a low mass close companion that could be responsible for the extreme RV shift of 107.0 \pm 22.0 km/s measured within only 31 min .