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in his two seminal papers , @xcite not only showed that two stars can exist in an envelope of a common equipotential and thus resolved the overall hitherto unexplainable properties of w  uma - type stars , but also very clearly indicated that such contact binaries must have very dissimilar components .
as pointed out by @xcite , evolution can create this dissimilarity .
the evolutionary state of contact binary stars remains unclear because their spectra can not be analyzed for abundances due to the extreme broadening and blending of spectral lines .
indirect information though , such as their progressively increasing numbers with age in old open @xcite and globular @xcite clusters , as well as the kinematic characteristics @xcite , very strongly suggest an advanced age of @xmath0 gyr .
recently , @xcite has developed a model with the currently less massive component being the more evolved one .
such a model is conceptually very close to that used to explain the semi - detached algols . in his model ,
the current secondary ( less - massive ) components must be in some cases very low in mass to explain systems like aw  uma or sx  crv @xcite .    in this letter we present a summary of results on the component masses ( for 112 systems ) and system angular momenta ( for 93 systems ) for low - temperature contact binaries .
the sample used was collected mainly from the list of contact binaries defined by @xcite .
half of the systems have solutions published in the frame of the w  uma project ( papers i - vi ) @xcite . all the rest
were collected from the literature , the physical parameters of which have been determined accurately , using photometric light curves and radial velocity measurements for both components . we also included the near - contact binaries ( ncbs ) and the detached close binaries ( dcbs ) listed in tables 2 and 3 of @xcite , in order to compare their physical parameters with those of our sample .
we had to exclude the cases , where the third light and the low - amplitude light variation give unreliable solutions , such as v2150  cyg , v899  her , ht  vir , bl  eri and go  cyg . in some cases ,
close binaries in triple systems have led to spurious solutions and for this reason they were excluded from our sample . other cases with third light contribution do not seem to produce any problem .
such systems , which are members in multiple systems , have better geometrical configuration and usually have very good spectroscopic determination of the third light contribution , allowing accurate determination of the orbital and physical parameters .
two systems , v351 peg and v402 aur were very probably incorrectly classified as w - type binaries , although the shape of their light curves does not support such a classification .
both systems have equal minima in their light curves , making it difficult to distinguish them from each other .
since their physical parameters ( masses and periods ) were closer to those of a - type binaries , they were classified as a - type systems .
only recently , good spectroscopic data has become available for more than a hundred contact binaries . since even small - mass secondaries
can be observed spectroscopically in contact systems , most objects have been analyzed as double - lined binaries bypassing any need of inferences based on single - lined data or solely on sometimes highly unreliable photometric elements ( particularly mass ratios ) .
in our study we consider component `` 1 '' as the more massive one .
our assumption is based on the double - lined spectroscopic observations , where the mass ratio is taken as @xmath1 .
in figure  [ fig1 ] we present the distribution of the component masses versus the orbital period .
it seems that the secondary components in all systems are very similar in masses , regardless of the orbital period .
masses of secondaries are between the values of 0 and 1 @xmath2 .
the mean value of the mass of the secondaries is @xmath3 , while the primaries have masses between 0.5 and @xmath4 ( only hv  uma has @xmath5 ) .
the same pattern of distribution appears when the masses of a and w - type systems are plotted in separate figures ( figures  [ fig2]a and  [ fig2]b ) . in this case , the average mass of a - type secondaries is equal to @xmath6 and that of w - type equal to @xmath7 .
it is remarkable to see the similarity of the mass distribution of secondary components with that of white dwarfs @xcite . according to @xcite
, mass exchange is taking place in the majority of contact binaries and the secondaries are helium - rich objects . in this case
the masses of the secondary components are expected to be similar or smaller than those of white dwarfs , which could have been grown as cores of isolated stars .
another interesting feature shown in figure  [ fig1 ] as well as in the upper and lower panels of figure  [ fig2 ] is that there is a total absence of systems with periods between 0.5 and 0.6 d. this gap is rather caused by the selection effect of our sample of contact binaries .
many contact binaries with equal minima and periods close to 12 h are difficult to be observed and can be mistaken as pulsating variables , with a period of 6 h ( i.e. @xmath8 variables ) , or remain unclassified . a recent study @xcite has also showed that many contact binaries are still undetected .    in our sample ,
all w - type systems have orbital periods shorter than 0.5 d , while the a - type systems can have all possible periods in the range considered , with a small preference in large values .
seven systems ( ck  boo , fp  boo , sx  crv , gr  vir , tz  boo , aw  uma and fg  hya ) ( especially aw  uma , sx  crv and tz  boo ) are plotted with triangles in all figures , as they are low - q systems with very low - mass secondaries ( @xmath9 ) . in these systems
the rotational angular momentum is mostly `` absorbed '' from the primary component and plays a significant role on the total angular momentum of the system .
according to @xcite these very low - q systems can not exist , since @xmath10 .
there is a co - existence of a and w - type systems with periods between 0.3 and 0.5 d. all systems with @xmath11 d are of w - type and all with @xmath12 d are of a - type .
a very interesting feature is shown in figure  [ fig3 ] , where the total mass of the above systems is plotted versus the period .
it is obvious that a - type systems are in general more massive than w - type ones . for comparison ,
in the same graph , we plotted the total mass of our cbs with the total mass of the short - period ( @xmath13 ) ncbs and dcbs , taken from @xcite .
one can see that most of the ncb and dcb systems have total mass similar to that of a - type contact binaries but larger than that of w - type contact binaries .
these observational facts strongly support the evolutionary progress from near - contact into contact configuration of a - type , without the need of mass loss from the system , as proposed by @xcite .
further progress in this direction , may transform the a - type systems into w - type , with a simultaneous mass loss , or can lead the a - type systems to become binaries with extreme small mass ratio .
the opposite direction would require the total mass to increase , which is unreasonable .
in figure  [ fig4 ] we present a plot of the orbital angular momentum versus the orbital period . in this plot
we see that a - type systems generally tent to have larger angular momenta than the w - type systems . on the top of each point in the graph
, a vertical line is added , representing the amount of the spin angular momentum of the two components .
the sum of the spin and orbital angular momenta will give the total angular momentum of each system .
in this way it can be seen that the angular momentum of close binaries can be studied with either the orbital or the total angular momentum .
the orbital angular momentum is not affected from observational or modeling errors , but only from masses and orbital periods . on the other hand ,
the spin angular momentum is affected from the errors in mass , radius and from the assumption we make for the radius of gyration , which is still under investigation @xcite .
for example , a slightly smaller radius of gyration would shift the seven low - q systems of our sample in a stable state .
figure  [ fig4 ] shows that formation of w - type systems from a - type is possible to be done , but the opposite direction is not strongly supported .
evolution from a - type to w - type systems seems to occure with a simultaneous mass and angular momentum loss , unless the evolutionary direction , after the ncbs evolve to contact systems , follows two separate tracks .
however , we can not exclude the possibility that some a - type systems with small angular momentum have evolved from w - type systems , while others ( with large angular momentum ) have evolved directly from ncbs @xcite .
do both a and w - type contact binaries have the same origin ? is the one type progenitor of the other ?
the above questions are still open for investigation .
the main result of the present study , taking into account the masses and angular momenta of contact binaries , is that the w - type systems can not produce a - type binaries , since angular momentum and mass can not be added to a system , but only lost from it .
it seems more reasonable that a - type systems evolve to w - type systems by loosing mass and angular momentum .
a similar evolutionary direction from long to short period binaries @xcite showed that systems with longer periods are kinematically young ( age 2 gyr ) in contrast to those with shorter periods ( age 8 gys ) .
an interesting result extracted from our plots is that the secondary components in close binary systems are very similar in mass .
scatter of the points in our plots is mainly due to inaccurate photometric and spectroscopic solutions and/or a possible undetected third light contribution .
since the best of the available data is used in our sample , only the third light could produce a problem and this is why some solutions are excluded from our sample .
a very recent study of @xcite about the formation of contact binaries in multiple systems , suggests that a large percentage of close binaries is formed into triple and multiple systems .
in such a case , a small amount of unreliability is added to all the solutions , if they are affected from an undetected third light .
our observational evidence ( figures 1 - 4 ) strongly supports the argument that the evolutionary process is from ncbs to a - type contact binaries , without any need of mass loss from the system .
the next step in this scenario may lead either to a transformation of a - type to w - type systems with a simultaneous mass and angular momentum loss , or to a - type systems with extremely low mass ratio .
these systems will eventually merge into a single , fast - rotating object .
the opposite direction of evolution seems to be impossible , since it requires an increase of the total mass and angular momentum of the system .
the authors gratefully acknowledge professor s. rucinski for his valuable help during the preparation of the manuscript and for many insightful discussions and new ideas about contact binary structure and evolution , as well as the anonymous referee for useful suggestions , which improved the article .
this project was supported by the special account for research grants no 70/3/7187 ( hrakleitos ) and 70/3/7382 ( pythagoras ) of the national and kapodistrian university of athens , greece .
rucinski , s. m. , 1998 , aj , 116 , 2998 rucinski , s. m. , 2000 , aj , 120 , 319 rucinski , s. m. , 2002 , pasp , 114 , 1124 stepien , k. , 2004 , in dupree a.k . ,
, eds , proc . iau symp .
219 , stars as suns : activity , evolution and planets .
, san fransisco , p. 967
results are presented on component masses and system angular momenta for over a hundred low - temperature contact binaries . 
 it is found that the secondary components in close binary systems are very similar in mass . 
 our observational evidence strongly supports the argument that the evolutionary process goes from near - contact binaries to a - type contact binaries , without any need of mass loss from the system . 
 furthermore , the evolutionary direction of a - type into w - type systems with a simultaneous mass and angular momentum loss is also discussed . 
 the opposite direction of evolution seems to be unlikely , since it requires an increase of the total mass and the angular momentum of the system .    
 [ firstpage ]    binaries : close - binaries : eclipsing - stars : evolution