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the gravities and temperatures of subdwarf b stars ( sdb ) suggest that they are composed of a helium core of 0.5m@xmath0 and a very thin hydrogen layer of @xmath10.02m@xmath0 .
although this has been known for some years it is still unclear how these stars lose most of their hydrogen envelopes and still manage to ignite helium .
both , single star and binary star evolution have been suggested as possible formation channels . in 2001 ,
maxted et al . found that a high fraction of sdb stars from the pg survey , 2/3 , are short period binaries .
the other third is thought to be made of long period binaries that formed through stable roche lobe overflow ( i.e. no common envelope ) and single stars ( green , liebert &  saffer 2001 ) .
these results indicate that formation via a binary evolution is one of the main channels to make sdb stars .
in this paper we present new results of the systematic search for sdb binary systems that we started a few years ago ( maxted et al .
2001 , morales - rueda et al . 2003 ) .
we also make use of the fact that short period sdb binaries do not change their orbital periods significantly after they emerge from the common envelope phase to test models of binary evolution .
we compare our results with those obtained by using population synthesis models by han et al .
( 2002 , 2003 ) .
we started this program in april 2000 and have been collecting data since then .
most of the spectra have been obtained with the intermediate resolution spectrograph ids on the 2.5 m int telescope and only in the last year we have started collecting spectra for southern hemisphere targets with the 1.9 m radcliffe telescope at saao .
the spectra taken on the first year of the program covered the h@xmath2 line . from august 2001 we obtained blue spectra instead , covering the balmer lines from h@xmath3 to h@xmath4 in most cases . by taking blue spectra we can not only look for radial velocity variations in the lines and measure orbital periods of the sdb binaries , but we can also fit the profiles of the lines by a grid of synthetic spectra and measure their effective temperatures , surface gravities and helium abundances ( saffer et al . 1994 ; napiwotzki 1997 ; heber , reid &  werner 2000 ) .
a complete description of the data taken with the int is given in morales - rueda et al .
our sample ( as of july 2003 ) contains 117 systems , 80 from the palomar - green ( pg ) survey , 16 from the kitt peak downes ( kpd ) survey and 12 from the edinburgh - cape ( ec ) survey .      to measure the radial velocities of the lines we perform least squares fitting of a model line profile that consists of 3 gaussians .
this fitting consists of several steps : * 1 .
* continuum normalise the spectra , * 2 .
* use default fixed values for the fwhms and heights of the 3 gaussians and fit all the spectra simultaneously to obtain the overall mean velocity , * 3 .
* fit all the lines simultaneously by fixing the overall velocity ( obtained in previous step ) and allowing the fwhms and heights of the 3 gaussians to vary * 4 .
* fit the spectra individually this time fixing the fwhms and heights to the values obtained in step 3 and allowing for the velocities to be variable , * 5 . *
shift out the velocities obtained in step 4 for each spectrum and obtain a more refined value for the fwhms and heights of the 3 gaussians and * 6 .
* fix the fwhms and heights to the refined values from step 5 and measure individual velocities for each spectrum .
we use as many balmer lines as there are present in each spectrum to perform the fitting . see fig .
[ fig1 ] for an example of fitting two balmer lines .
( 100,0)(10,20 ) ( 0,0 )      for the red spectra ( up to august 2001 ) the statistical uncertainties are of the order of 1km / s whereas for the blue spectra the uncertainties are of the order of 2km / s ( a tenth of a pixel ) .
we know that there are also other unaccounted sources of error that are probably not correlated with the orbit or the statistical errors , i.e. slit - filling , intrinsic variability of the star .
we estimate these systematic errors by assuming that when added in quadrature with the raw uncertainties they will give us a reduced @xmath5 = 1 .
we assume that in all cases the systematic uncertainty is at least 2km / s ( @xmath6 a tenth of a pixel ) and this value works well for most systems . in some cases
the systematic error turns out to be larger , with the largest value being 5km / s for kpd0025 + 5402 .
we fit the data with a model composed of a sinusoid and a constant ( cumming , marcy &  butler 1999 ) to determine the orbital periods .
this method works better than the lomb - scargle periodogram for small numbers of points . for each fit
we calculate its associated @xmath5 and select the solution with the minimum @xmath5 value .
an example of a periodogram and a radial velocity curve folded on the orbital period are presented in fig.[fig2 ] .    to distinguish between competing aliases ( those separated by @xmath7 20 ) we use a technique developed by marsh , dhillon &  duck ( 1995 ) and later adapted to this study by morales - rueda et al .
( 2003 ) to calculate the probability of the true orbital period being further than 1 and 10 per cent from the value obtained .
we assume our values are correct if the probability of the period being wrong is @xmath8 0.1 per cent .
( 100,0)(10,20 ) ( 0,0 )    ( 0,0 )
in fig .  [ fig3 ]
we present the number of sdb binaries known when this paper was produced versus their orbital periods .
the systems included in the figure are those presented in fig .
4 of morales - rueda et al .
( 2003 ) plus 7 new ones from our sample . during this workshop
a few more sdb binaries were presented by edelmann et al .
which have not been included in the figure .
( 100,0)(10,20 ) ( 0,0 )    the picture we now have of how the orbital periods of sdb binaries are distributed is very different from what it was before we started this program .
we can see that the distribution extends toward longer periods than it was initially thought .
another important feature is the excess of systems at orbital periods of the order of 1 day ( the 1 day bin actually includes systems with orbital periods between 0.32 and 1 days ) .
these systems should be more difficult to detect as one requires more observations to be able to distinguish between the orbital period and the observational aliases
. this feature has been seen by other authors ( green , private communication ) .
thanks to the long time baseline of our observations we are starting to detect binary systems with longer orbital periods , i.e. 4 systems with orbital periods of tens of days and the first one with a period of hundreds of days .
maxted et al .
discuss somewhere else in these proceedings the nature of the companions to these sdbs in detail .
it is worth pointing out here that we find 2 systems in our sample with m dwarf companions , pg1017 - 086 ( maxted et al .
2002 ) and pg1329 + 159 ( maxted et al . in preparation ) .
19 others are found to have white dwarf ( wd ) companions ( maxted et al . in preparation ) .
han et al . ( 2002 , 2003 ) present five possible channels for the formation of sdb stars , three of them will form sdbs in binary systems .
[ fig4 ] shows the orbital period distribution predicted by their favoured model .
the plot also presents our data in the form of vertical ticks on the horizontal axis .
most of the binaries in our sample seem to have formed via the second common envelope ( ce ) ejection channel ( short period binaries with wd companions ) , some via the first ce ejection channel ( short to intermediate periods with main sequence companions ) .
their models predict the formation of a large number of long period binaries via the first roche lobe overflow ( rlof ) channel ( first suggested by tutukov &  yungleson 1990 ) . up to this day
we have only detected one of such system with an orbital period of a few hundred days in our sample .
we seem to be missing a very large population of binaries with large orbital periods .
( 100,0)(10,20 ) ( 0,0 )
( 100,0)(10,20 ) ( 0,0 )    our sample suffers from several important biases :    * \1 .
targets have been selected from 3 main surveys , pg , kpd and ec .
the pg survey is biased against companions of g and k spectral type as any target with a spectrum showing caii h - lines was taken off the survey .
this should be taken into account when looking at the companions of sdbs as most of the ones from the pg survey will be wds instead of main sequence stars .
this has important consequences when one intends to compare binary formation models with observations .
it is difficult to assess how important this bias is .
a second important bias is given by the resolution of the spectra .
up to now we have used intermediate resolution spectrographs to measure radial velocities .
longer period systems will show smaller amplitude radial velocities and we are probably missing them .
the use of higher resolution spectrographs might change significantly the picture we have at the moment . *
\3 . only now , after almost four years of observations , is the time baseline getting long enough to detect the longer period systems .
this can explain the dearth of long period systems compared to han et al.s ( 2003 ) predictions .
all targets we have observed are bright ( v @xmath9 14.5 ) .
it is also well known that the pg survey is incomplete at bright magnitudes due to saturation of the plates .
all targets are in the disk of the galaxy ( see fig .
[ fig5 ] ) .
the more sdbs we study the more binaries we find , the fraction we find being of the order of 2/3 in agreement with maxted et al .
most of the binaries found seemed to have formed via the first and second ce ejection phases but we are starting to find systems that might have formed through the first rlof as predicted by han et al .
( 2003 ) .
the int is operated on the island of la palma by the ing in the spanish observatorio del roque de los muchachos of the iac .
we thank patt for their support of this program .
the radcliffe telescope is operated by the south african astronomical observatory .
we present the results of a 3.5 year long campaign to measure orbital periods of subdwarf b ( sdb ) star binaries . 
 we directly compare our observed orbital period distribution with that predicted by using binary population synthesis . up to now , 
 most of our systems seem to have been formed through two of the formation channels discussed by han et al.(2003 ) , i.e. the first and the second common envelope ejection ( ce ) channels . at this point , thanks to the long baseline of our observations , we are starting to detect also very long orbital period systems . 
 these have probably come from a complete different formation path , the first stable roche lobe overflow ( rlof ) channel in which the first mass transfer phase is stable . 
 this channel is expected to lead to the formation of very wide binaries with typical orbital periods ranging from one month to one year .