The interacting binary CX Draconis: New orbital elements and evidence for mass transfer

New improved parameters for both components of the double-lined spectroscopic Be binary CX Dra are presented. CX Dra (P = 6.69603 d) consists of a detached primary (B2.5V, 7.3 M., 4 R.) and a secondary (F5III, 1.7 Mo) probably filling in its Roche lobe. The inclination angle i = 50 degrees and dimensions of both components do not allow the eclipses to occur. The equatorial rotational velocity of the primary 210 Km s(-1) is highly asynchronous. CX Dra can be therefore classified as a high-mass non-eclipsing Algol-type binary. Emission lines of H alpha and HeI 6678 were analysed. The V/R ratio of the double peaked emission in the original spectra of H alpha smoothly varies with the orbital phase. The difference profiles constructed by subtracting the synthetic spectra of both components from the observed spectra enabled studying the non-photospheric emission (presumably originating in the circumstellar matter). The non-photospheric emission in H alpha is interpreted in terms of two distinct components. The broad part following the RV curve of the primary belongs probably to the accretion disk while the other component (narrow peak) cannot be linked with any star. The velocity field of the narrow peak offers a possibility that this feature is connected with the mass stream. The broad double-peaked emission attributed to the accretion disk around the hot primary is visible also in HeI 6678. The temperature of the disk must therefore correspond at least to 11000-12000 K and this disk is significantly hotter than in most other Algol-type systems. Conspicuous secular changes of the emission in HeI 6678 were revealed. The analysis presented here led to the conclusion that the Be phenomenon in CX Dra can be explained by the mass transfer in the interacting binary.