You are an expert at summarizing long articles. Proceed to summarize the following text: 
dao white dwarfs , the prototype of which is hz34 @xcite , are a group of white dwarfs that are observed to have both hydrogen and helium lines in their optical spectra , in contrast to the more common das , which exhibit only hydrogen absorption . hence , the presence of detectable he must arise from the existence of a thin overlying h envelope or there must be a mixing process , dredging up he from the deeper layers of the stellar photosphere . radiative forces appear to be too low to support sufficient quantities of helium to be able to produce the observed lines @xcite . a thin h envelope might arise through float up of residual h in a he - rich do atmosphere as objects transfer between the helium and hydrogen cooling channels @xcite . however , the discovery by @xcite that the heii  line in the optical spectra of one dao was better reproduced by a homogeneous rather than a layered atmospheric model was contrary to this view . @xcite  performed a similar comparison on a sample of 14 daos , but found only one object for which a stratified model was preferred . of the remaining objects , most were found to be comparatively hot and low gravity , implying they have low mass . therefore , the progenitors of some of these daos are unlikely to have had sufficient mass to ascend the asymptotic giant branch ( agb ) , and instead they may have evolved from extended horizonal branch stars . it was suggested that , in these stars , weak mass loss may be occurring , a process that might be able to support the observed quantities of helium in the line forming regions of the daos @xcite . three objects ( re1016 - 053 , pg1413 + 015 and re2013 + 400 ) were in close binary systems and have m dwarf ( dm ) companions . these have ` normal ' temperatures and gravities , yet still have detectable helium lines in their optical spectra . this may be because mass is lost as the progenitor star passes through the common envelope phase , leading to the star being hydrogen poor , and allowing a weak process , such as mass loss , to mix helium into the line forming region of the white dwarf . alternatively , these daos might be accreting from the wind of their companions - re0720 - 318 , another dao with a dm companion , has been shown to have changing helium abundance over time , which might be due to episodic accretion @xcite . in addition , @xcite  identified likely spatial non - uniformities in the surface abundance of helium , which are consistent with models of accretion . finally , for one star in the @xcite  sample ( pg1210 + 533 ) , the heii  line profile was not reproduced satisfactorily by either chemically homogeneous or stratified models . in addition , its helium line strengths have been observed to change over a time period of @xmath015 years @xcite . this is not a high temperature , low gravity object @xcite , and has no infrared excess that might indicate a companion star , so it does not appear to fit in with any of the other daos . therefore , dao white dwarfs form a relatively heterogeneous group . for some , the existence of cool , non - degenerate companions may play a significant role in the presence of photospheric he , either from prior common envelope evolution or from ongoing mass transfer . we investigate a sample of 16 dao white dwarfs to determine how many exhibit evidence of short term radial velocity variations and thus may be members of close binary systems , using _ far - ultraviolet spectroscopic explorer _ ( _ fuse _ ) data . _ fuse _  spectra are built up from a number of exposures that last up to 90 minutes , which need to be combined to achieve the desired signal - to - noise of the proposer . although the signal - to - noise of these exposures is far worse than in the final combined spectrum , strong interstellar and photospheric absorption features can still be identified . this investigation exploits this fact to search for radial velocity shifts in photospheric lines relative to the stationary interstellar medium lines , which might indicate the presence of short period binary systems . far - ultraviolet ( far - uv ) data for all the objects were recorded by the _ _ fuse__spectrographs . table [ fuseobs ]  lists the observations that were used , which were downloaded by us from the multimission archive ( http://archive.stsci.edu/mast.html ) , hosted by the space telescope science institute . overviews of the mission and in - orbit performance have been published by @xcite and @xcite respectively ; below is a brief description of the instrument .    [ cols="<,<,^,^,^,^,^",options="header " , ]     of the daos with comparitively high mass , evidence for binarity was found for re0720 - 318 and re2013 + 400 ( both of which are dao+dm binaries ) , but not for pg1210 + 533 and lb2 . only two exposures were obtained for pg1210 + 533 , while lb2 has 5 , and it might be possible that radial velocity shifts were missed because the separation in time of exposures was insufficient , because the system was viewed at an unfavourable phase , or if the radial velocity measurement technique were insufficiently sensitive . in addition , if a system is at high inclination , no radial velocity variations would be detectable . this may indeed be the case for lb2 , which was identified by @xcite  as having an infrared excess . it should also be noted that for this object the mass determined from the lyman line results is lower than that obtained from the balmer line results and using the lower mass would place lb2 in the lower mass group . as shown in figure [ probs ] , since there are fewer exposures for pg1210 + 533 , the method is less sensitive than for lb2 . however , it is still 80%  likely that radial velocity shifts would be detected by this experiment if it were in a binary system with period less than almost a day , unless its period happened to match the separation of exposures in time , or a multiple of that separation . this indicates that this method is quite a sensitive way of searching for white dwarfs in binary systems , particularly if a number of exposures have been made , and hence it is unlikely , although not impossible , that pg1210 + 533 is in a short period binary system , hence the explanation for helium in the spectrum of pg1210 + 533 is still elusive .    in the observations of the remaining 12 white dwarfs , evidence of binarity was found for only ton320 , pg0834 + 500 , hs1136 + 6646 and feige55 , which are all known or suspected binary stars . although low mass white dwarfs can be formed within in binary systems , these results suggest that not all of these have companions . therefore , for most of the high temperature , low gravity , low mass objects , evolution within a binary system , or accretion from a companion , can not be used to explain their low mass or the presence of helium absorption lines in their optical spectra , and therefore evolution as single objects from the extended horizontal branch is still the most likely explanation for their existence . _ fuse _  data have been used to measure the radial velocity variations in 16 dao white dwarfs , from the position of the photospheric heavy element absorption lines in their spectra . the technique was found to be successful in detecting radial velocity variability in all the known binaries , and even with only 2 exposures there is better than a 80%  chance of observing velocity changes if the period of the system is less than a day , apart from where the period is a multiple of the temporal separation of exposures . radial velocity variations were only detected for those objects already known or suspected to be binaries , although it is possible that some of the others possess binary companions that are too highly inclined , or which were observed at unfavourable phases or have orbital periods too long with respect to the time span covered by our observations to be detected . of the low mass daos , significant changes in radial velocity were observed for only four of the twelve objects , suggesting that the majority of this type of dao do not exist in binary systems and may therefore have evolved as single stars from the extended horizontal branch . no evidence of binarity was found for two of the four higher mass white dwarfs in the sample . one of these objects , lb2 , has an infrared excess and hence may in fact have a companion . the presence of helium in the spectrum of the other , pg1210 + 533 , is still unexplained . based on observations made with the nasa - cnes - csa far ultraviolet spectroscopic explorer . _ fuse _  is operated for nasa by the johns hopkins university under nasa contract nas5 - 32985 . sag , mab , mrb and pdd were supported by pparc , uk ; mrb acknowledges the support of a pparc advanced fellowship . jbh wishes to acknowledge support from nasa grants nag5 - 10700 and nag5 - 13213 . barstow m.  a. , et al . , 1995 , mnras , 272 , 531 barstow m.  a. , odonoghue d. , kilkenny d. , burleigh m.  r. , fleming t.  a. , 1995 , mnras , 273 , 711 bergeron p. , wesemael f. , beauchamp a. , wood m.  a. , lamontagne r. , fontaine g. , liebert j. , 1994 , apj , 432 , 305 bloecker t. , 1995 , a&a , 297 , 727 dobbie p.  d. , barstow m.  a. , burleigh m.  r. , hubeny i. , 1999 , a&a , 346 , 163 driebe t. , schoenberner d. , bloecker t. , herwig f. , 1998 , a&a , 339 , 123 finley d.  s. , koester d. , basri g. , 1997 , apj , 488 , 375 fontaine g. , wesemael f. , 1987 , iau colloq . 95 : second conference on faint blue stars , 319 - 326 good s.  a. , barstow m.  a. , holberg j.  b. , sing d.  k. , burleigh m.  r. , dobbie p.  d. , 2004 , mnras , 355 , 1031 heber u. , dreizler s. , hagen h .- j . , 1996 , a&a , 311 , l17 hillwig t.  c. , honeycutt r.  k. , robertson j.  w. , 2000 , aj , 120 , 1113 holberg , j.b . and magargle , 2005 , in proceedings of the nato advanced workshop on white dwarfs 2004 holberg j.  b. , saffer r.  a. , tweedy r.  w. , barstow m.  a. , 1995 , apj , 452 , l133 koester d. , weidemann v. , schulz h. , 1979 , a&a , 76 , 262 kruk j.  w. , chayer p. , dupuis j. , 2003 , in proceedings of the nato avanced workshop on white dwarfs 2002 , de martino , d. , silvotti , r. , solheim , j .- e . and kalytis , r. ( eds ) , kluwer moos h.  w.  et al . , 2000 , apj , 538 , l1 morton d.  c. , 1991 , apjs , 77 , 119 napiwotzki r. , schberner d. , 1993 , in : barstow m. ( ed ) 8th european workshop on white dwarfs , kluwer academic publishers , p.99 pounds k.  a. , et al . , 1993 , mnras , 260 , 77 saffer r.  a. , livio m. , yungelson l.  r. , 1998 , apj , 502 , 394 sahnow d.  j.  et al . , 2000 , apj , 538 , l7 sing , d.  k. , holberg , j.  b. , burleigh , m.  r. , good , s.  a. , barstow , m.  b. , oswalt , t.  d. , howell , s.  b. , brinkworth , c.  s. , rudkin , m. , johnston , k. , rafferty , s. , 2004 , aj , 127 , 2936 unglaub k. , bues i. , 1998 , a&a , 338 , 75 unglaub k. , bues i. , 2000 , a&a , 359 , 1042 vennes s. , pelletier c. , fontaine g. , wesemael f. , 1988 , apj , 331 , 876 vennes s. , thorstensen j.  r. , 1994 , apj , 433 , l29 vennes s. , thorstensen j.  r. , polomski e.  f. , 1999 , apj , 523 , 386 wesemael f. , greenstein j.  l. , liebert j. , lamontagne r. , fontaine g. , bergeron p. , glaspey j.  w. , 1993 , pasp , 105 , 761
we report on a search for evidence of binarity in _ far - ultraviolet spectroscopic explorer _  ( _ fuse _ ) observations of dao white dwarfs . 
 spectra recorded by _ fuse _  are built up from a number of separate exposures . 
 observation of changes in the position of photospheric heavy element absorption lines between exposures , with respect to the stationary interstellar medium lines , would reveal radial velocity changes - evidence of the presence of a binary system . 
 this technique is successful in picking out all the white dwarfs already known to be binaries , which comprise 5 out of the sample of 16 , but significant radial velocity shifts were found for only one additional star , ton320 . 
 this object is also known to have an infrared excess @xcite . 
 daos can be separated broadly into low or normal mass objects . 
 low mass white dwarfs can be formed as a result of binary evolution , but it has been suggested that the lower mass daos evolve as single stars from the extended horizontal branch @xcite , and we find no evidence of binarity for 8 out of the 12 white dwarfs with relatively low mass . the existence of higher mass daos can also be explained if they are within binary systems , but of the four higher mass stars in the sample studied , pg1210 + 533 and lb2 do not exhibit significant radial velocity shifts , although there were only two exposures for the former object and the latter has an infrared excess @xcite . 
 [ firstpage ]    stars : atmospheres - white dwarfs - ultraviolet : stars .