Direct calibration of the Cepheid period-luminosity relation

After the first release of Hipparcos data, Feast & Catchpole gave a new val ue for the zero-point of the visual Cepheid period-luminosity relation, bas ed on trigonometric parallaxes. Because of the large uncertainties on these parallaxes, the way in which individual measurements are weighted is of cr ucial importance. We therefore conclude that the choice of the best weighti ng system can be aided by a Monte Carlo simulation. On the basis of such a simulation, it is shown that (i) a cut-off in pi or in sigma(pi)/pi introdu ces a strong bias; (ii) the zero-point is more stable when only the brighte st Cepheids are used; and (iii) the Feast & Catchpole weighting gives the b est zero-point and the lowest dispersion. After correction, the adopted vis ual period-luminosity relation is [M-V] = -2.77 log P - 1.44 +/- 0.05, More over, we extend this study to the photometric I band (Cousins) and obtain [ M-I] = -3.05 log P - 1.81 +/- 0.09.