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it is well - known that the fate of a star after it has evolved away from the main sequence ( ms ) depends on its mass . the accepted scenario for the post - ms evolution of low to intermediate mass stars is that , following the helium flash , burnout of he occurs in the core on the horizontal branch . after evolution up the asymptotic giant branch , the star sheds its outer envelope , which is illuminated as a planetary nebula ( pn ) by the still - hot stellar core . however , in as many as 20% of cases ( blcker @xcite ) the star , as it evolves towards the white dwarf ( wd ) region of the hr diagram , re - ignites a residual helium shell in a vltp and retraces its evolutionary track to the right to become a born again red giant ( bag ; see herwig @xcite and references therein ) . the final evolution to a wd is predicted to take roughly a few centuries , thus representing a very rapid ( and hence seldom seen ) phase of stellar evolution . a very small number of stars is known to have undergone vltps , e.g. sakurai s object ( v4334  sgr ) , v605  aql and fg  sge . all are c - rich , h - poor , have extensive dust shells , and lie at the centre of a pn . observations of bags in the past @xmath0  years have shown that the rate at which the final evolution occurs has been seriously _ underestimated . _ we have a long - standing programme of observations of bags at infrared ( ir ) wavelengths , some extending at least over a couple of decades . these included ground - based observations ( e.g. woodward , @xcite ; tyne , @xcite ; gehrz , @xcite ) and observations using the  infrared spectograph ( e.g. evans  @xcite ) and the forcast grism spectrometer on the stratospheric observatory for infrared astronomy ( ) . fg  sge underwent its vltp in 1880 . the 536  spectrum as observed by  and  is shown in fig . there is clearly a dust continuum , at a temperature @xmath1  k. this is considerably lower than that determined by either woodward  ( @xcite ) or gehrz  ( @xcite ) , who found that there was a major dust - formation event sometime between 1983 and 1992 . they also found that , over the period 19922001 , the dust temperature was consistently within @xmath2  k of 1000  k , indicating that mass - loss and dust production was continuing . more recent  data show that , sometime between 1992 and 2006 ( the date of the first  observation ) mass loss had ceased and that the dust shell had started cooling . of interest is the difference between the underlying 650  k blackbody and the  irs spectrum , shown in fig .  [ bags ] . there is clear evidence for hydrocarbon `` unidentified infrared '' ( uir ) emission , and absorption by acetylene ( c@xmath3h@xmath3 ; 13.688 ) ; this demonstrates that the mass lost in the 19922001 episode ( gehrz , @xcite ) was carbon - rich . also highlighted are two weak emission features at 33.45  and 34.75  ( see below ) . the  forcast observation , obtained in 2014 , show that the dust has further cooled ( see fig . [ bags ] ) . the vltp of sakurai s object occurred in 1996 . thorough analyses of the ir development and the implications for the evolution of the ( carbon ) dust produced are given by tyne  ( @xcite ) and hinkle & joyce ( @xcite ) . this object also showed c@xmath3h@xmath3 in absorption , as well as hcn and a range of polyynes ( hc@xmath4n ; evans  @xcite ) . the ejection of an optically thick carbon dust shell commenced in late 1997 ( evans  @xcite ; hinkle & joyce @xcite ) ; following the dust ejection event the central star has been completely ( @xmath5 ) obscured , and has only recently ( 2015 february ) become visible at @xmath6 . exquisite observations by chesneau  ( @xcite ) with the mid - ir interferometer midi / vlti showed that the dust was confined to a disc . the cooling of the dust shell from 620  k to 180  k , as observed from the ground ( ukirt+jcmt ) and from space- and air - borne ( , ) observations , is shown in fig . the hcn isotopologues in the  irs spectra show that the @xmath7c/@xmath8c ratio is @xmath9 , consistent with that deduced from the fundamental and first overtone co absorption ( eyres , @xcite ; pavlenko , @xcite ) , and with its vltp status . the nature of ck  vul is unclear . once thought to be the oldest `` old nova '' ( shara , @xcite )  it erupted in 1670  it has also been suggested that it is a bag ( evans , @xcite and references therein ) , a diffusion - induced nova ( miller bertolami , @xcite ) , and a stellar merger ( kaminski , @xcite ) . the  irs spectrum , of both the central object and the inner nebula , shows a number of uir features on a 170  k blackbody continuum ; the difference between the irs spectrum of the central object , and a 170  k blackbody is shown in fig . there are a number of ionic features ( e.g. [ o ] at 25.89 ) and we again see uir features that are completely different from those commonly seen in novae ( helton , @xcite ) . we also note the two features at 33.45  and 34.75 , also present in fg  sge ( see above ) . they are labelled as being due to [ s ] and [ si ] in fig . [ bags ] but they are extraordinarily strong in ck  vul compared to their strength in fg  sge . there seems to be no reasonable way in the case of ck  vul to account for these features in terms of ionic emission only . it may be that they arise either in entirely different environments in the two objects , or in different carriers . observations of the bag phenomenon at mid - to - far ir ( 538 ) wavelengths provide a wealth of information ( e.g. about the mineralogy of the dust , far ir fine structure lines )  and hence about the circumstellar environment of bags  that can be obtained by no other means . observations of bags using  are continuing and will be reported elsewhere .    99 blcker , t. , 2001 , ap&spsci , 275 , 1 chesneau , o. , , 2009 , a&a , 493 , l17 evans , a. , , 2002 , mnras , 332 , l35 evans , a. , , 2006 , mnras , 373 , l75 eyres , s. p. s , , 2004 , mnras , 350 , l9 gehrz , r. d. , , 2005 , apj , 623 , 1105 helton , l. a. , , 2011 , in _ pahs and the universe _ , eas publ . series , vol . 46 , 407 herwig , f. , 2005 , ara&a , 43 , 435 hinkle , k. h. , & joyce , r. r. , 2014 , apj , 785 , 146 miller bertolami , m. m. , , 2011 , mnras , 415 , 1396 pavlenko , ya . v. , , 2004 , a&a , 417 , l39 ; kaminski , t. , , 2015 , nature , 520 , 322 shara , m. m. , , 1985 , apj , 294 , 271 tyne , v. h. , , 2002 , mnras , 334 , 875 woodward , c. e. , , 1993 , apj , 408 , l37
we describe the evolution of the carbon dust shells around very late thermal pulse ( vltp ) objects as seen at infrared wavelengths . 
 this includes a 20-year overview of the evolution of the dust around sakurai s object ( to which olivier made a seminal contribution ) and fg  sge . 
 vltps may occur during the endpoint of as many as 25% of solar mass stars , and may therefore provide a glimpse of the possible fate of the sun .