Photometry and RVs of BL Her stars

Using the surface brightness-colour relation version of the Baade-Wesselink technique, we determined the projection factors and radii of eight nearby BL Her type stars. We adopted accurate distances of target stars from Gaia Data Release 3. Time series photometry in the V and KS bands have been collected with two telescopes located at the Rolf Chini Cerro Murphy Observatory (former Cerro Armazones Observatory), while spectroscopic data have been obtained within dedicated programmes with instruments hosted by the European Southern Observatory.

The measured projection factors for the stars with good quality data are in the range between 1.21 and 1.36. The typical uncertainty of projection factors is 0.1. The mean value is 1.330+/-0.058, which gives the uncertainty of ~4%.

The main sources of uncertainty on the p-factors are statistical errors of the Baade-Wesselink fit (related to the dispersion and coverage of light and radial velocity curves) and parallax. In the case of radii, the biggest contribution to the error budget comes from the Ks band photometry's systematic uncertainty and parallax. The determined radii allowed us to construct the period-radius relation for BL Her stars.

Our period-radius relation is in good agreement with the previous empirical calibration, while two theoretical calibrations found in the literature agree with our relation within 2{sigma}. We also confirm that BL Her and RR Lyr stars obey an apparent common period-radius relation.