Hipparcos Parallaxes of the Nearest Cepheids { Implicative of Binarity

The period{luminosity relation of Cepheids is calibrated on the basis of the Hipparcos trigonometric parallax values, using data on Cepheids within one kiloparsec. It is shown that the milliarcsec-size apparent orbit of Cepheids belonging to binary systems has an unfavourable innuence on the parallax determination. About two third of the sample studied here have physical companion(s). The true value of the parallax of such stars, as well as the projected orbit of the individual Cepheids have to be extracted from the original Hipparcos measurements. It is promising, however, that the P{L relation based on the trigono-metric parallax of solitary Cepheids is in agreement with the recently determined other versions.