The Angular Size of the Cepheid ℓ Carinae: A Comparison of the Interferometric and Surface Brightness Techniques

Recent interferometric observations of the brightest and angularly largest classical Cepheid, ℓ Carinae, with ESO's Very Large Telescope Interferometer have resolved with high precision the variation of its angular diameter with phase. We compare the measured angular diameter curve to the one that we derive by an application of the Baade-Wesselink-type infrared surface brightness technique and find a near-perfect agreement between the two curves. The mean angular diameters of ℓ Car from the two techniques agree very well within their total error bars (1.5%), as do the derived distances (4%). This result is an indication that the calibration of the surface brightness relations used in the distance determination of far-away Cepheids is not affected by large biases.