The intermediate-band approach to the surface-brightness method for Cepheid radii and distance determination

The surface-brightness parameter F-nu is calibrated in terms of the Stromgr en intermediate-band colour b - y. The relation F-nu - (b - y)(o) valid for Cepheids is calibrated using accurate near-infrared radii and distances fo r selected Cepheids. We have obtained uvby photometry for non-Cepheid giant and supergiant stars with known angular diameters and compared the slope a nd zero-point of their F-nu - (b - y)(o) relation with the Cepheid calibrat ion. We found that the two calibrations are significantly different. The th eoretical models lie in between the two calibrations. It is remarked that F -nu-colour relations derived from non-Cepheids and involving blue colours ( e.g. B - V or b - y) are not applicable to Cepheids, while those involving redder colours (e.g. V - R, V - K or V - J) also produce good radii for Cep heids. Selected Cepheids as calibrators lead to the accurate relation F-nu = 3.898(+/- 0.003) - 0.378(+/- 0.006)(b - y)(o), which allowed the calculat ion of radii and distances for a sample of 59 Galactic Cepheids. The uncert ainties in the zero-point and slope of the above relation are similar to th ose obtained from near-infrared colours, and determine the accuracies in ra dii and distance calculations. While infrared light and colour curves for C epheids may be superior in precision, the intermediate-band b-y colour allo ws the recovery of mean radii with an accuracy comparable to those obtained from the infrared solutions. The derived distances are consistent within t he uncertainties with those predicted by a widely accepted period-luminosit y relationship. Likewise, the resulting period-radius relation from the int ermediate-band approach is in better agreement with infrared versions than with optical versions of this law. It is highlighted that the intermediate- band calibration of the surface-brightness method in this work is of compar able accuracy to the near-infrared calibrations. The present results stress the virtues of uvby in determining the physical parameters of supergiant s tars of intermediate temperature.