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we used a 60 cm cassegrain telescope equipped with an emi  9558b photomultiplier ( 1991    1999 ) , a rca  c31034 photomultiplier ( 2001    2004 ) and ccd camera ( 2005  -  2008 ) , to obtain the @xmath0 photometry of the selectd symbiotic stars ( for details see * ? ? ? * ) . for tx cvn and ag peg we used additional data from the slovak long - term monitoring programme of photometric observations of selected symbiotic stars ( * ? ? ?
* ; * ? ? ?
* and references therein ) .
all multicolour photometric measurements of zz  cmi are presented in figure  [ zzcmi ] ( left panel ) .
the fast fourier transform ( fft ) of the data gives a 218.59  days period for @xmath1 filters and 835.12 days    for u ( figure  [ zzcmi ] middle panel ) . on the right panel of figure  [ zzcmi ]
the @xmath1 light curves phased with the double period ( 437.18  days ) are demonstrated .
we have used wilson    devinney s code @xcite to verify the assumption , that the double sine wave could be interpret as an ellipsoidal effect connected with the tidal distortion of the giant surface . a circular orbit ( @xmath2 ) with inclination @xmath3 was adopted .
we have determined the ephemeris @xmath4 , and used it to phase the data .
the temperature of the cool component was assumed to be @xmath5 according to m5iii classification of @xcite .
many solutions for the components mass , in the range @xmath6 for the giant and @xmath7 for the white dwarf , were tested .
filters data of zz  cmi , right panel : the residual @xmath1 and u light curves of zz  cmi phased with 437.18 and 835.12 days respectively .
residual magnitudes were obtained by substraction of a mean value from observational data ( see figure  [ zzcmi ] , left panel ) for each of the photometric systems.,scaledwidth=50.0% ]     and @xmath8 is presented . the filled area shows the possible mass solutions for which the cool giant does not overfill the roche surface and the mass of the white dwarf is smaller than @xmath9 , right panel : the @xmath10 light curves phased with double of 218.59 days period .
solid lines represent the synthetic curves computed for @xmath11 , @xmath12.,title="fig : " ]   and @xmath8 is presented .
the filled area shows the possible mass solutions for which the cool giant does not overfill the roche surface and the mass of the white dwarf is smaller than @xmath9 , right panel : the @xmath10 light curves phased with double of 218.59 days period .
solid lines represent the synthetic curves computed for @xmath11 , @xmath12.,title="fig : " ]    the possible solutions , in which the hot star is less massive than @xmath13 and the giant does nt overflow roche lobe , are restricted to the filled area in figure  [ zzcmims ] ( left panel ) .
it is clearly seen that the cool component has to be less massive than @xmath14 and the mass ratio ( q ) is greater than @xmath15 .
all synthetic light curves obtained for combination of masses from the filled region in figure  [ zzcmims ] ( left panel ) give practically the same solutions , similar to that shown in figure  [ zzcmims ] ( right panel ) .
tx cvn is symbiotic star , which spectrum is combined by a late b and an early m star spectra @xcite and with an orbital period @xmath16 days @xcite .
spectrophotometric observations shows that the hot component is not a normal late _ b - type _ star , because the he  i features are produced in an expanding with a velocity of @xmath17 photosphere .
we used photometric data to confirm the spectroscopic period of this system . to subtract the long term trends we fitted lines to our data ( figure  [ txcvn ] left panel ) , and then the fft was calculated separately for the _ ubvri _ data .
the power spectra are shown in the middle panel of figure  [ txcvn ] .
the periodograms show a peak at frequency @xmath18 , which corresponds to the known period of @xmath19 days .
the phased light curves are shown in figure  [ txcvn ] ( right panel ) .
filters data of tx  cvn .
right panel : the @xmath0 light curves of tx  cvn  the residual data after the trend subtraction phased with the 199.75 days period .
the @xmath20 light curve shows evidence for an elliptical effects .
the triangles mark the slovak data , the circles - our data . ]
ag  peg is a symbiotic binary consisting of a white dwarf and a red giant .
the stars orbiting their mass center with a period of @xmath21 days @xcite . @xcite found the evidence for colliding stellar winds in this system : a fast wind ( @xmath22 ) from the hot component and a slow wind ( @xmath23 ) from the giant .
the colliding winds form the gaseous nebula surrounding the system .
our @xmath0 photometric measurements of this star are presented in figure  [ agpeg ] ( left panel ) .
the periodograms of multicolour photometry are shown in figure  [ agpeg ] ( middle panel ) .
we adopted a mean value of 806.31  days for the period in all filters obtained from the fft analysis .
this value is within the error box of the period published by kenyon et al .
the data were phased with our period , using @xmath24 from @xcite .
the phased light curves are shown in figure  [ agpeg ] ( right panel ) .
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we present ubvri photometry of three symbiotic stars zz cmi , tx cvn and ag peg carried out from 1997 to 2008 in piwnice observatory near toru . to search orbital periods of these stars 
 fourier analysis was used . for two of them , tx cvn and ag peg , 
 we have confirmed the earlier known periods . for zz 
 cmi we found a relatively short period 218.59  days . assuming , that the orbital period is twice longer ( p=437.18  days ) , the double sine wave in the light curve can be interpreted by ellipsoidal effect