For determining distance and mass of pulsating stars a new , purely photometric method is described in which radial velocity observations are not needed .
From multicolour photometry the variation of angular diameter is determined in a conventional way by using the surface brightness of the theoretical atmospheric models ATLAS of Kurucz ( [ ] ) .
As a function of phase the following two parameters are introduced in the Navier-Stokes equation : gravity ( of the appropriate static ATLAS model ) , and angular diameter .
Distance and mass are derived from phases of standstill .
Conclusions are drawn on the hydrodynamic behaviour of the atmosphere .
The new method is compared with the Baade-Wesselink ( BW ) method .
As an example the RR Lyrae variable SU Dra is given : using UBVR _ { C } 
photometry , a distance of 647 \pm 16 pc , and a mass of 0.66 \pm . 03 { \cal M } _ { \sun } 
were found .
In addition to the radius change ( 4.58 - 5.51 ) R _ { \sun } 
radius undulations have been found with amplitude \approx 0.2 R _ { \sun } 
and period P / 5 , which are synchronized with the main period P = 0 \aas@@fstack { d } 66042 .
The present method has confirmed the distance value from the BW method ; thus , in the problem of different distance scales from BW and other methods the short extragalactic distance scale has been bolstered by an independent argument .
The mass is some 30 percent larger than the value from the equation of pulsation of van Albada & Baker ( [ ] ) .