Continued radial velocity observations over several years, have allowed to discover four spectroscopic binary systems among our sample of twenty five galactic Cepheids. Each system is composed by the Cepheid and an invisible probably hot dwarf star. A spectroscopic orbit has been determined for each system. The orbital periods for RX Cam, MW Cyg, Z Lac and U Vul are 1113, 439, 383 and 2510 days, respectively. For the pulsation motion, radial velocity versus phase diagram have been fitted by an analytic relation, formally similar to a Kepler motion. The stellar radius variation of each cepheid has been derived by integration of this relation over the whole pulsating period.