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Paper text:
as it was pointed out in one of the papers of @xcite , massive professional photometric surveys are today sources of many , often unexpected , discoveries ( e.g. the asas project of @xcite or the kepler mission of @xcite ) . on the other hand , databases collecting amateur photometric measurements are growing rapidly , as telescopes and ccd detectors are easily accessible nowadays @xcite .
this gives us access to a number of valuable open astronomical archives , both professional and amateur , with a richness of data gathered during the past decades .
we have now a great opportunity to trace photometric behaviour of a lot of variable objects , for instance dwarf novae stars .
dwarf novae are cataclysmic binaries with a white dwarf as a primary and a low mass main sequence star , filling its roche lobe , as a secondary .
due to the transfer of mass from the secondary onto the non - magnetic primary , an accretion disk is formed .
such a mass transfer leads to outbursts in dwarf novae , as they are caused by sudden gravitational energy release due to accretion of this material onto the white dwarf .
dwarf novae with shortest orbital periods ( @xmath0 ) are called su  uma stars .
they are characterised by the presence of superoutbursts , next to normal outbursts , which are about @xmath1 brighter and about ten times less frequent than normal outbursts @xcite .
the length of the supercycle , i.e. the time between two successive superoutbursts , is one of the most fundamental properties of su  uma stars and is specific for each of them .
the nature of this feature is still uncertain .
nowadays there is an open debate about the cause of superoutbursts .
the thermal - tidal instability ( tti ) model @xcite suggests that the sypercycle length ( @xmath2 ) is set by the mass transfer rate ( @xmath3 ) .
according to this scenario , the observed @xmath2 is inversely proportional to @xmath3 .
the tti model shows that normal outbursts remove less material from the disk than it has been accreted since the last normal outbursts .
this makes the disk grow during a number of normal outbursts .
when the disk reaches the critical radius ( @xmath4 , with @xmath5 being the binary separation ) , it becomes eccentric and tidal effects occur , which causes superoutburst , during which the disk is shrinking again .
although the tti model is commonly accepted , it still seems to need some improvements . for instance the tti simulations which reproduced the light curve of one of the most active dwarf novae , er  uma star , required an artificial increase of the mass transfer rate by the factor of ten in comparison to values expected from the theory based on gravitational radiation @xcite .
another example is the fact shown by ( * ? ? ?
* and references therein ) that embedded precursors of superoutbursts are present also in systems with long orbital periods .
this argues for a more general model , which is not restricted to the mass ratio of @xmath6 , as the tti model .
recently @xcite presented evidence in favor of this model .
although their analysis was based on variations of the negative superhump period of a single dwarf nova , v1504 cyg , they claimed that the cause of superoutbursts is finally revealed .
however , @xcite showed that this object can not be considered as a representative for all systems which show negative superhumps .
he also presented a number of arguments which suggest that superoubursts are caused by strongly enhanced mass transfer ( emt ) rate @xcite .
recent observations @xcite surprisingly showed that the supercycle length of one of the most active dwarf novae , ix  draconis , has been increasing with a constant rate since the last twenty years .
this is interesting in the context of evolution of such systems .
encouraged by this fact , we decided to investigate the same issue for other well - observed active objects of the su  uma type .
the paper is arranged as follows . in section  [ sec - data ]
we give information on the data used in this research .
the analysis is presented in section  [ sec - analysis ] .
results together with discussion are given in section  [ sec - res ] .
we summarize the main conclusions in section  [ sec - conc ] .
[ cols="^,^,^,^,^,^,^,^",options="header " , ]     [ tab - nextsteps ]
we analysed a set of photometric data covering last decades of observations for the most active su  uma stars in order to study changes of their supercycle lengths .
despite the common feature of the selected objects , which is the highest activity in this class of stars , they are diverse in terms of their orbital periods below the period gap . for all of the analysed objects we found positive values of period derivatives of their supercycle lengths .
there are some subtle fluctuations for short time scales in the behaviour of @xmath2 , but the general trend is the same in each case .
increasing supercycle lengths mean that the mean mass transfer rates have been decreasing for these objects over the last decades .
this is in agreement with the scenario of the evolution of bk  lyn presented by @xcite , which seems to be a general case .
this phenomenon is important in the context of evolution of such systems .
we performed a detailed analysis of extensive photometric observations of a sample of most active dwarf novae , that is su  uma stars which are characterised by supercycle lengths shorter than 120  days . 
 we found the observational evidence that supercycle lengths for these objects have been constantly increasing over the past decades , which indicates that their mean mass transfer rates have been decreasing during that time . 
 this seems to be a common feature for this type of stars . 
 we present numerical results in each case and estimate time scales of future development of these systems . 
 this study is important in the context of evolution of dwarf novae stars and perhaps other cataclysmic variables .    
 binaries : close  stars : cataclysmic variables , dwarf novae , evolution