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the aim of our work is to separate geometrical phenomena ( e.g. eclipses , disk precession ) from physical phenomena which occur in two prototype dwarf novae : su uma and u gem . in particular , we study the influence of such phenomena on flickering variability observed in our multicolor light curves of these systems .
the flashes are observed with the mean spacing 15 days for the normal outbursts and about 260 days for superoutbursts in the case of su uma . only normal outburst with mean spacing about 100 days are observed in the case of u gem .
our observations were collected during three distinct phases of activity : quiescence , outburst and superoutburst . for su uma ,
these are most probably the first observations obtained in all phases of activity using one 4-band photometric system .
we performed the observations from jan 2006 to feb 2008 using a 60 cm cassegrain telescope at the astronomical observatory of the nicolaus copernicus university in piwnice , near torun , poland .
we observed in @xmath1 , @xmath2 , @xmath3 and @xmath4 bands and in a white light mode . in the current paper
we do not present @xmath2 and @xmath3 light curves .
exposure times for the su uma observations were 40 sec and 30 sec in @xmath1 and @xmath2 bands , respectively , and 20 - 25 sec in @xmath3 and @xmath4 bands .
the resultant time resolution was approximately 95 sec .
the exposure times used for the u gem observations were 20 - 60 sec in @xmath1 , 20 - 40 sec in @xmath2 , 10 - 20 sec in @xmath3 and 5 - 15 sec in @xmath4 , with time resolution of 135 sec .
for the observations collected in the white light mode for both stars , the exposure time was set to 5 sec which resulted in a time resolution of 8 sec .
the level of accuracy of our measurements was 0.01 mag . in order to phase our data , we adopted the following ephemeris from the literature : + su uma : @xmath5 = 2446143.6627 + 0.076351 @xmath6 @xmath7 ( thorstensen 1986 ) + u gem : @xmath8 = 2437638.8270 + 0.17690619 @xmath6 @xmath7 ( krzemiski 1965 , marsh 1990 ) . + where @xmath5 concerns the spectroscopic inferior conjunction of emission lines source for su uma , and @xmath8 is the mideclipse moment for u gem .
light curves of su uma obtained without filter and in @xmath1 and @xmath4 bands as well as @xmath9 colour indicies are shown in figure  1 . the corresponding light and
colour curves of @xmath10 gem are presented in figure 2 .
we do not show our observations during outburst of u gem because they do not cover the whole orbital period .
we noticed fast flickering and orbital changes of the brightness during the quiescence phase in su uma and u gem systems ( figure 1 and 2 ) .
especially both unfiltered light curves show almost identical pattern of these variations during whole orbital period , if we neglect the eclipses in u gem . the higher amplitude of such variations in the case of su uma is a result of the dominant influence of hot spot over the accretion disk and the other components of the system on the light curve .
moreover , the unfiltered observations during the quiescence and the @xmath4 observations during the outburst phase , indicate an additional slow pattern of changes on timescales compatible with the orbital period .    during the superoutburst phase , characteristic superhumps are apparent in the light curves of su uma .
@xmath9 color index is constant for both superoutburst and outburst phases , which suggests that the changes of the system geometry are responsible for the observed superhumps .
it was discussed by udalski ( 1990 ) that the superhumps are the result of disk precession .
the maxima of superhamps during superoutburst occur in different phases than orbital maxima during quiescence .
the preccesion in the orbital motion direction causes their period of changes to be longer than orbital period .
highly time resolved observations in a non - filter mode in the quiescence show flickering variability on timescales of minutes and amplitudes of 0.05 - 0.1 mag in u gem ( figure 2a ) and even 0.7 mag in su uma ( figure 1a ) .
the flickering amplitude in both stars increases toward the shorter wavelenghts during the quiescence and almost disappear during the outburst and the superoutburst phases in su uma .
the maximum amplitude corresponds to the best visibility of the hot spot ( face - on ) near phase 0.6 . on the other hand , in the case of u gem flickering
is well visible not only when the hot spot is well seen ( figure  2a ) .
the flickering is most pronounced near phase 0.4 when the hot spot does not face the observer .
thus , the influence of intrinsic accretion disk instabilities seem to be a very important source of flickering in u gem
. +   * acknowledgements : * this work was supported by the polish mnisw grant n203 018 32/2338 .
krzemiski w. , 1965 , _ apj _ , * 142 * , 1051 - 1067 marsh t.r .
, keith horne 1990 , _ aj _ , * 364*,637 - 646 thorstensen j. r. , wade r. a. , oke j. b. 1986 , _ apj _ , * 309 * , 721 - 731 udalski a. , 1990 , _ aj _ , * 100 * , 26 - 232
the results of time - resolved observations of su uma and u gem obtained over two - years are presented . 
 both stars are prototypes of different classes of dwarf novae . 
 we studied brightness variations on different time scales : orbital , qpo and flickering . 
 the multicolor @xmath0 photometry allows to distinguisch the geometrical and physical sources of these variations .