CEPHEIDS AND BINARITY .2. RADIAL-VELOCITI ES AND SPECTROSCOPIC ORBITSOF 4 GALACTIC CEPHEIDS - RX-CAMELOPARDALIS, MW-CYGNI, Z-LACERTAE AND U-VULPECULAE

Continued radial velocity observations over several years, have allowe d to discover four spectroscopic binary systems among our sample of tw enty five galactic Cepheids. Each system is composed by the Cepheid an d 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 fitt ed by an analytic relation, formally similar to a Kepler motion. The s tellar radius variation of each cepheid has been derived by integratio n of this relation over the whole pulsating period.