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Due to their high intrinsic brightness, type II supernovae (SN) can be usedas lighthouses to constrain distances in the Universe using variants of theBaade-Wesselink method. Based on a large set of CMFGEN models (Hillier & Miller1998) covering the photospheric phase of type II SN, we study the variousconcepts entering one such technique, the Expanding Photosphere Method (EPM).We compute correction factors $\xi$ needed to approximate the syntheticSpectral Energy Distribution (SED) with that of a blackbody at temperature $T$.Our $\xi$, although similar, are systematically greater, by $\sim 0.1$, thanthe values obtained by Eastman et al. (1996). This translate into a systematicenhancement of 10-20% in EPM-distances. We find that line emission andabsorption, not directly linked to color temperature variations, canconsiderably alter the synthetic magnitude, and cause above-unity $\xi$-values.Optically-thick lines, present in the optical at late-times, can also introducea strong wavelength-dependence of the photospheric radius, invalidating the useof the Baade method. Both the impact of line-blanketing on the SED and thephotospheric radius at low $T$ suggest that the EPM is best used at earlytimes, when the outflow is fully ionized and line-opacity mostly confined tothe UV range. We also investigate how reliably one can measure the photosphericvelocity from P-Cygni line profiles. Contrary to the usually held belief, thevelocity at maximum absorption in the P-Cygni trough of optically-thick linescan both {\it overestimate or underestimate} the photospheric velocity, with amagnitude that depends on the SN outflow density gradient and the opticalthickness of the line.Comment: 16 pages, 19 figures, accepted for publication in A&

Distance determinations using type II supernovae and the expanding photosphere method