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in the standard picture ( e.g.  ) subdwarf b ( sdb ) stars are core helium burning stars on the extended horizontal branch ( ehb ) . they consist of a he core with a mass of @xmath00.48@xmath1 , the canonical mass for a he core flash on the first red giant branch ( rgb ) , and a thin , inert hydrogen shell . evolutionary models of ehb stars calculated by and adopted an enhanced mass loss on the rgb , however , without specifying a particular mechanism . recent radial velocity ( rv ) surveys revealed that a large fraction of all sdb stars resides in close binaries (; ; ) . @xcite ( @xcite , @xcite ) performed a theoretical investigation of possible close binary channels for the formation of sdbs : stable roche lobe overflow , common envelope ejection , and merging of two he core white dwarf ( wds ) . they concluded that it is possible to explain _ all _ sdb stars as the result of close binary evolution . the helium - rich subdwarf o stars ( he - sdo ) are hotter than sdbs , but have similar surface gravities . their formation is still a mystery . in one scenario they are explained as an advanced stage of post - ehb evolution ( , ) . another scenario proposes the formation of he - sdos via a phase as extreme he stars after the merging of two he core wds ( cf . discussion in ) . spy , the eso * s*upernovae type ia * p*rogenitor surve*y * ( @xcite @xcite , @xcite ) , is a programme dedicated to search for short period binary wds ( double degenerates  dds ) . the aim of spy is the detection of dd progenitors of supernovae type ia by means of a survey for rv variations . snia progenitor candidates should be close enough to merge within one hubble time due to gravitational wave radiation and the combined mass should exceed the chandrasekhar limit for wds . the spy input catalogue collected wd candidates brighter than @xmath2 from a variety of source catalogues ( cf .  ) . the sdbs analyzed in this paper were selected from the hamburg / eso survey @xcite , the wd catalogue , and the hamburg quasar survey @xcite .    as a by - product , spy produced high accuracy rv measurements of more than 46 sdb stars and 23 he - sdo stars , which were included in our sample because of misclassifications in the input catalogues . thus spy produces a large sample of known close binary sdb stars , offering an independent data set and allowing for a substantial improvement of the statistics . the he - sdo sample included in spy is the first he - sdo sample systematically checked for rv variations . this allows us to investigate the role of binarity for their formation and their possible link with the sdb stars . spy is carried out with the high - resolution spectrograph uves at the ut2 telescope of eso vlt . with our instrument setup we achieve nearly complete spectral coverage from 3200  to 6650with only two @xmath080   wide gaps . since spy was implemented as a bad weather program , we used a wide ( @xmath3 ) slit to minimize slit losses . the resulting spectral resolving power is @xmath4 ( 0.36  at h@xmath5 ) or better , if seeing disks were smaller than the slit width . due to the nature of the project , two spectra at different `` random '' epochs separated by at least one day are observed ( cf .   , for details ) . the rv variations were measured differentially between both observed spectra with a cross correlation routine . this allowed to include all suitable lines ( h , he , and metal lines ) for the rv determination in a very flexible way . we routinely achieved a rv accuracy of 2km / s or better . rv variations could be checked for 46 single - lined sdbs from the complete spy sample ( , @xcite ) . their distribution in the temperature - gravity plane is displayed in fig . [ f : hr ] . our rv measurements yielded 18 rv variable binaries , i.e.  39% of this sample . this indicates a high fraction of close binaries in the spy sample , conforming the importance of binary channels for the formation of sdbs . however , somewhat surprisingly our result points to a lower fraction of detections than in the sample , who found 58% rv variable sdbs .    for a quantitative comparison with other samples and an evaluation of the true fraction of binaries one needs to know the detection efficiency , i.e.   the chance that a binary escaped detection , because of unfavorable inclination angles or phasing of the observations . the detection efficiency obviously depends on the orbital period and the mass of the unseen companion . we performed a monte carlo simulation of our observed sample for two different choices of the mass of the invisible companion : @xmath6 , the mass of a typical wd , and @xmath7 for a typical low mass main sequence star . for every star we calculated detection probabilities taking into account the timing of exposures and the rv accuracy . results for the sample were coadded yielding the effective sample size ( i.e.  the size of an equivalent sample with 100% binary detection rate ) plotted in fig . [ f : detection ] as function of orbital period . for an alternative interpretation we indicate the relative percentage of the detection rate . the detection probabilities are quite high for short period systems , exceeding 90% for systems with a wd companion and periods shorter than one day . .corrections for detection efficiency for the sdb and the he - sdo samples . calculations were done for the two distributions discussed in the text , the canonical sdb mass of @xmath8 , and different companion masses . [ cols= " < , < , < , < , < , < " , ]     the very low number of close binaries in our he - sdo sample provides evidence that he - sdos , as a class , are not successors of ehb stars . formation of he - sdos by merging of two he core wds would be in agreement with our finding .    how do our sdb results compare with the binary population synthesis of ? @xcite produced several simulated samples of sdbs resulting from their binary evolution channels with different parameter choices and presented a best fit sample ( their simulation set  2 ) . they could reproduce the properties of observed sdb binaries ( from ) and the distribution in the temperature - gravity plane ( cf .  , @xcite ) . observational selection effects against sdbs with visible cool companions ( gk selection ) and against stars above the ehb ( strip selection ) were simulated . they predict an observable binary frequency for their best fit model of 45% ( the remainder resulting from merging ) , which compares well to our 40% , if we apply the same criteria to the spy sample .    however , two problems remain . simulated a population with thin disk characteristics , which should be representative for the sample , but this sample yielded a binary frequency of 69% . on the other hand predict a low binary frequency for stars above the ehb , because this region should contain many sdbs resulting from merging . although a systematic observational investigation of stars in this region has still to be done , it appears that the binary frequency in this region is higher than expected ( see fig . [ f : hr ] ) from the theoretical simulation . a further complication might arise if some or all of the mergers produce he - sdos , instead . in summary the new theoretical simulations represent the properties of the observed sdb sample well , but some unsolved problems remain . future investigations will help to decide , if these can be solved by minor adjustments or imply larger revisions . ahmad , a. & jeffery , c.s . : 2003 , _ a&a _ 402 , 335 dcruz , n.l . , dorman , b. , rood , r.t . , & oconnell , r.w . : 1996 , _ apj _  466 , 359 dorman , b. , rood , r.t . , & oconnell , r.w . : 1993 , _ apj _  419 , 596 dreizler , s. , heber , u. , werner , k. , moehler , s. , de boer , k.s . : 1990 , _ a&a _  235 , 234 edelmann , h. , heber , u. , lisker , t. , green , e.m . : 2003 , _ ap&ss _ , these proceedings hagen , h .- j . , groote , d. , engels , d. , & reimers , d. : 1995 , _ a&as _   111 , 195 han , z. , podsiadlowski , p. , maxted , p.f.l . , marsh , t.r . , & ivanova , n. : 2002 , _ mnras _  336 , 449 han , z. , podsiadlowski , p. , maxted , p.f.l . , & marsh , t.r . : 2003 , _ mnras _  341 , 669 heber , u. : 1986 , _ a&a _  155 , 33 lisker , t. , heber , h. , napiwotzki , r. , et al . : 2003a , _ ap&ss _ , these proceedings lisker , t. , heber , h. , napiwotzki , r. , et al . : 2003b , _ a&a _ , submitted maxted , p.f.l . , heber , u. , marsh , t.r . , & north , r.c . : 2001 , _ mnras _  326 , 1391 mccook , g.p . & sion , e.m . : 1999 , _ apjs _ 121 , 1 morales - rueda , l. , maxted , p.f.l . , marsh , t.r . , north , r.c . , & heber , u. : 2003 , _ mnras _  338 752 napiwotzki , r. , christlieb , n. , drechsel , h. , et al . : 2001a , _ an _  322 , 411 napiwotzki , r. , edelmann , h. , heber , u. , et al . : 2001b , _ a&a _  378 , l17 napiwotzki , r. , christlieb , n. , drechsel , h. , et al . : 2003 , _ eso messenger _ 112 , 25 saffer , r.a . , livio , m. , & yungelson , l.r . : 1998 , _ apj _  502 , 394 wesemael , f. , winget , d.e . , cabot , w. , van horn , h.m . , & fontaine , g. : 1982 : _ apj _  254 , 221 wisotzki , l. , koehler , t. , groote , d. , & reimers , d. : 1996 , _ a&as _   115 , 227
we present the results of a radial velocity ( rv ) survey of 46 subdwarf b ( sdb ) and 23 helium - rich subdwarf o ( he - sdo ) stars . 
 we detected 18 ( 39% ) new sdb binary systems , but only one ( 4% ) he - sdo binary . 
 orbital parameters of nine sdb and sdo binaries , derived from follow - up spectroscopy , are presented . 
 our results are compared with evolutionary scenarios and previous observational investigations .