A new approach to the surface brightness method for Cepheid radii determination.

Photometric data in the uvby- Str6mgren system have been obtained for a sample of 127 Cepheids. Presently, light and color curves for 72 Cepheids are available. A calibration of the surface brightness parameter Fv has been obtained in terms of the intrinsic color (b - y)o. This opens the possibility of calculating Cepheid radii from intermediate band photometry. The non-simultaneity of photometric and radial velocity data can be overcome by properly shifting the light, color and radial velocity curves. Cepheid distances are adopted from P-L predictions which leave the radius as the only free parameter. Radii are reported for 54 Cepheids and the implied Period-Radius relation is discussed.