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A photometric calibration of Kurucz static model atmospheres is used to obtain the following parameters of RR Lyrae stars: variation of stellar angular radius  ϑ , effective temperature  T  e , gravity  g  e  as a function of phase, interstellar reddening   E ( B − V )  towards the star and atmospheric metallicity  M . Photometric and hydrodynamic conditions are given to find the phases of pulsation when the quasi‐static atmosphere approximation (QSAA) can be applied. The QSAA is generalized to a non‐uniformly moving spherical atmosphere, and the distance  d , mass  and atmospheric motion are derived from the laws of mass and momentum conservation. To demonstrate the efficiency of the method, the   UBV ( RI )  C    photometry of SU Dra was used to derive the following parameters:  [ M ] =−1.60 ± 0.10  dex, , equilibrium luminosity   L  eq = 45.9 ± 9.3  L ⊙  and  T  eq = 6813 ± 20  K .

Fundamental parameters of RR Lyrae stars from multicolour photometry and Kurucz atmospheric models – I. Theory and practical implementation